Patent Publication Number: US-11662550-B1

Title: Systems and methods for varifocal adjustments

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/905,432, titled “SYSTEMS AND METHODS FOR VARIFOCAL ADJUSTMENTS,” filed 25 Sep. 2019, the entire disclosure of which is incorporated herein by reference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying figures illustrate a number of example embodiments and are a part of the specification. Together with the following description, these figures demonstrate and explain various principles of the present disclosure. 
       FIG.  1    is an illustration of a user wearing a head-mounted display (“HMD”) system, with certain portions and elements of the HMD system removed or shown as transparent to view underlying and internal elements, according to at least one embodiment of the present disclosure. 
       FIG.  2    is a top view of optical assemblies of the HMD of  FIG.  1   , according to at least one embodiment of the present disclosure. 
       FIG.  3    is a schematic side view of an optical assembly of an HMD, according to at least one embodiment of the present disclosure. 
       FIG.  4    is a perspective view of an optical assembly of an HMD including a flexure assembly, according to at least one embodiment of the present disclosure. 
       FIG.  5    is a detailed perspective view of the flexure assembly of  FIG.  4   , according to at least one embodiment of the present disclosure. 
       FIG.  6    is a perspective view of a lens movement mechanism including a flexure assembly, according to at least one embodiment of the present disclosure. 
       FIG.  7    is a perspective view of a lens movement mechanism including a flexure assembly, according to at least one additional embodiment of the present disclosure. 
       FIG.  8    is a perspective view of a lens movement mechanism including a flexure assembly, according to at least one further embodiment of the present disclosure. 
       FIG.  9    is a perspective view of a lens movement mechanism including a flexure assembly, according to at least one other embodiment of the present disclosure. 
       FIG.  10    is a perspective view of a lens movement mechanism including bearing elements, according to at least one embodiment of the present disclosure. 
       FIGS.  11  and  12    are a detailed perspective views of portions of the lens movement mechanism of  FIG.  10   . 
       FIG.  13    is a perspective view of a lens movement mechanism, according to at least one additional embodiment of the present disclosure. 
       FIG.  14    is a perspective view of one example configuration of a bearing assembly of the lens movement mechanism of  FIG.  13   . 
       FIG.  15    is a cross-sectional view of another example configuration of a bearing assembly of the lens movement mechanism of  FIG.  13   . 
       FIG.  16    is a detailed perspective view of a lens movement mechanism, according to at least one additional embodiment of the present disclosure. 
       FIG.  17    is a partially cut-away perspective view of a lens movement mechanism, according to at least one further embodiment of the present disclosure. 
       FIG.  18    is a detailed perspective view of a lens movement mechanism, according to at least one other embodiment of the present disclosure. 
       FIG.  19    is a partially cut-away perspective view of a lens movement mechanism including a lens movement stop mechanism, according to at least one embodiment of the present disclosure. 
       FIG.  20    is front view of the lens movement mechanism and the lens movement stop mechanism of  FIG.  19   . 
       FIG.  21    is a detailed perspective view of the lens movement stop mechanism of  FIG.  19   . 
       FIG.  22    is a detailed front view of the lens movement stop mechanism of  FIG.  19   . 
       FIG.  23    is another detailed perspective view of the lens movement stop mechanism of  FIG.  19   . 
       FIG.  24    is a perspective view of an optical assembly including shock protection elements, according to at least one additional embodiment of the present disclosure. 
       FIG.  25    is a cross-sectional view of the optical assembly of  FIG.  24   . 
       FIG.  26 A  is a perspective view of an optical assembly including a shock protection element, according to at least one additional embodiment of the present disclosure. 
       FIG.  26 B  is a partial cross-sectional view of the optical assembly of  FIG.  26 A . 
       FIG.  27    is a partial cross-sectional view of an optical assembly including a shock protection element, according to at least one further embodiment of the present disclosure. 
       FIG.  28    is a flow diagram illustrating a method of varying at least one optical property of an optical lens system, according to at least one embodiment of the present disclosure. 
       FIG.  29    is a flow diagram illustrating a method of fabricating a display subassembly, according to at least one embodiment of the present disclosure. 
       FIG.  30    is a flow diagram illustrating a method of making varifocal adjustments, according to at least one embodiment of the present disclosure. 
       FIG.  31    is an illustration of example augmented-reality glasses that may be used in connection with embodiments of this disclosure. 
       FIG.  32    is an illustration of an example virtual-reality headset that may be used in connection with embodiments of this disclosure. 
    
    
     Throughout the figures, identical reference characters and descriptions indicate similar, but not necessarily identical, elements. While the example embodiments described herein are susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the appendices and will be described in detail herein. However, the example embodiments described herein are not intended to be limited to the particular forms disclosed. Rather, the present disclosure covers all modifications, equivalents, and alternatives falling within this disclosure. 
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Varifocal systems may be useful in a variety of devices, including eyeglasses, cameras, and artificial-reality (e.g., virtual-reality, augmented-reality, hybrid-reality, or mixed-reality) systems. For example, in artificial-reality systems, a head-mounted display (“HMD”) may present stereoscopic digital images to a user&#39;s eyes to provide the impression of three-dimensional (“3D”) objects and scenes. The images may be projected through one or more lenses. Varifocal systems may enable a focal distance of the images to be adjusted, such as to counteract or supplement natural focal changes in the user&#39;s eyes as the user observes the images at different perceived 3D distances. 
     The present disclosure is generally directed to systems and methods for varifocal adjustments. In some embodiments, the present disclosure includes systems and methods for moving a first lens of a lens pair to adjust a distance between the first lens and a second lens of the lens pair, which may alter at least one optical property (e.g., a focal distance) of the lens pair. The first lens may be moved substantially along an optical axis of the first lens. For example, the first lens may be coupled to a flexure assembly and/or a ball-bearing assembly, which may act as a guide for movement of the first lens between two or more positions. In some examples, the flexure assembly and/or ball-bearing assembly may constrain movement of the first lens to a substantially linear pathway. In some embodiments, the movement of the first lens may include some off-axis tilting, which may be predictable due to the configuration of the flexure assembly and/or ball-bearing assembly. This predictability may enable compensation of displayed images to be achieved, such as through software-driven alterations of the displayed images. 
     The following will provide, with reference to  FIGS.  1 - 9   , detailed descriptions of systems for varifocal adjustments that include flexure assemblies for guiding movement of a movable optical lens. With reference to  FIGS.  10 - 12   , the following will provide detailed descriptions of systems for varifocal adjustments that include ball-bearing assemblies for guiding movement of a movable optical lens. With reference to  FIGS.  13 - 18   , the following will provide detailed descriptions of lens movement mechanisms including bearing assemblies. With reference to  FIGS.  19 - 23   , the following will provide detailed descriptions of lens movement stop mechanisms for stopping movement of a movable optical lens. With reference to  FIGS.  24 - 27   , the following will provide detailed descriptions of various optical assemblies including shock protection elements. With reference to  FIGS.  28 - 30   , the following will provide detailed descriptions of various methods related to systems for varifocal adjustments. With reference to  FIGS.  31  and  32   , the following will provide detailed descriptions of various artificial-reality systems that may include or be implemented with systems for varifocal adjustments. 
     Referring to  FIG.  1   , a representation of a user  100  wearing an HMD system  102  is illustrated. Portions of the HMD system  102  are shown as transparent or are omitted for a clear view of underlying elements. As illustrated in  FIG.  1   , the HMD system  102  may be a virtual-reality HMD system  102 , although concepts of the present disclosure are also applicable to other types of systems that may benefit from varifocal adjustments. The HMD system  102  may include a right optical assembly  103 A for displaying visual content to a user&#39;s right eye and a left optical assembly  103 B for displaying visual content to the user&#39;s left eye. The right optical assembly  103 A and the left optical assembly  103 B are collectively referred to as optical assemblies  103 . An electronic display  104  may be included in each of the right optical assembly  103 A and the left optical assembly  103 B. The electronic display  104  is shown in  FIG.  1    in the right optical assembly  103 A, but a similar electronic display is omitted from the left optical assembly  103 B of  FIG.  1    for a better view of underlying components. In additional embodiments, a single electronic display  104  may be shared between the right optical assembly  103 A and the left optical assembly  103 B. 
     The HMD system  102  may include a frame  106  on which the two optical assemblies  103  are respectively mounted. The optical assemblies  103  may be configured for displaying digital images (e.g., stereoscopic images) to the eyes of the user  100 . Each of the optical assemblies  103  may include an optical lens pair  110  including a movable first lens  112  and a stationary second lens  114  (“movable” and “stationary” being with reference to the frame  106 ) and the electronic display  104  (e.g., a digital display element, such as an LED display, an LCD display, and OLED display, etc.). The first lens  112  may, in some embodiments, be positioned between the second lens  114  and the electronic display  104 . The first lens  112  may have a first optical axis  116  and the second lens  114  may have a second optical axis  118 . As illustrated in  FIG.  1   , in some embodiments the first optical axis  116  and the second optical axis  118  may be substantially collinear. 
     In some examples, the term “substantially” in reference to a given parameter, property, or condition may mean and include to a degree that one of ordinary skill in the art would understand that the given parameter, property, or condition is met with a small degree of variance, such as within acceptable manufacturing tolerances. By way of example, depending on the particular parameter, property, or condition that is substantially met, the parameter, property, or condition may be at least 90% met, at least 95% met, at least 99% met, or fully met. 
     Each of the optical assemblies  103  may include a flexure assembly  120 . The flexure assembly  120  may be configured to guide movement of the first lens  112  relative to the frame  106 . The flexure assembly  120  may include at least one flexure element  122 . For example, a first movable end portion  124  of the at least one flexure element  122  may be coupled to the first lens  112  (e.g., to a peripheral edge of the first lens  112 ), such as via a bracket  125 . A second fixed end portion  126  (“fixed” being relative to the frame  106 ) may be coupled to the frame  106 . One or more flexure elements  122  may extend between the first movable end portion  124  and the second fixed end portion  126  of the flexure assembly  120 . 
     As illustrated in  FIG.  1   , in some examples the at least one flexure element  122  may include more than one flexure element  122 , such as a first flexure element  122 A and a second flexure element  122 B. The first flexure element  122 A may be positioned adjacent to a first lateral side (e.g., an upper side) of the first lens  112  and the second flexure element  122 B may be positioned adjacent to a second, different lateral side (e.g., a lower side) of the first lens  112 . The presence and configuration of the two flexure elements  122 , which may be positioned substantially symmetrical about a centerline passing through the first lens  112 , may further facilitate the movement of the first lens in the desired direction (e.g., substantially along the optical axis of the first lens) while inhibiting unwanted movement. The flexure elements  122  may be positioned to be outside of the viewing area of the user  100 , such that a view of light traveling along (e.g., substantially parallel to) the optical axes  116 ,  118  and through the optical lens pair  110  from the electronic display  104  may be unimpeded by the flexure element  122 . 
     The first flexure element  122 A may include a first end portion  124 A that is movable relative to the frame  106  and a second end portion  126 A that is fixed relative to the frame  106 . Likewise, the second flexure element  122 B may include a first end portion  124 B that is movable relative to the frame  106  and a second end portion  126 B that is fixed relative to the frame  106 . Each of the flexure elements  122  may be configured to bend to allow the first lens  112  to move along the first optical axis  116  relative to the frame  106  and relative to the second lens  114 . Movement of the first lens  112  relative to the second lens  114  may alter at least one optical property (e.g., a focus) of the optical lens pair  110 , as further explained below with reference to  FIG.  3   . By way of example and not limitation, the first lens  112  may be movable relative to the second lens  114  and relative to the frame  106  over a distance of up to about 4 mm. 
     The flexure elements  122  may also serve as a guide to constrain movement of the first lens  112  to a particular movement profile. In some examples, the flexure elements  122  may be configured to maintain substantially linear movement of the first lens  112 , such as along the first optical axis  116 . Any deviation from linear movement (e.g., a rotational movement, a torsional movement, and/or a translational movement) may be predictable based on the geometry and configuration of the flexure elements  122 . Such predictable deviations, if present, may be accounted for by the HMD system  102 , such as by counteracting any change in optical properties (e.g., optical distortions) with software that may make corresponding alterations to an image displayed on the electronic display  104 . 
     Each of the flexure elements  122  may be or include a substantially planar element when in a resting (e.g., unbent) state. A central portion of each of the flexure elements  122  may be stiffened, such as by providing the central portion with a greater relative thickness than end portions thereof. For example, a stiffener material may be adhered, welded, brazed, overmolded, fastened, or otherwise coupled to each of the flexure elements  122 . In additional examples, a unitary, integral material may be molded, extruded, machined, or otherwise formed to include a central portion with a higher thickness (and consequently a higher stiffness) than end portions thereof. The flexure assembly  120  may include at least two flexure elements  122  that are spaced apart from each other by a gap to provide for relative ease of bending in a desired direction (e.g., about an axis extending parallel to and across a width of a major plane of the flexure elements  122 ) and relative stiffness in a torsional direction (e.g., about an axis extending along a length of the flexure elements  122 ). This configuration may facilitate movement of the first lens  112  in a desired direction (e.g., substantially along the first optical axis  116  of the first lens  112 ) while inhibiting (e.g., reducing or eliminating) other movement, such as off-axis rotation (e.g., twisting). Example configurations of and details regarding the flexure assembly  120  and flexure elements  122  thereof are described below. 
     The first and second lenses  112 ,  114  may be, for example, Fresnel lenses, convex lenses, concave lenses, or a combination thereof. A material of the lenses  112 ,  114  may include a polymer material (e.g., polycarbonate), a glass material, a crystalline material, or any other suitable optical lens material. In some embodiments, one or more of the lenses  112 ,  114  may have one or more coatings, such as anti-reflective coatings. 
       FIG.  2    illustrates a top view of the optical assemblies  103 A and  103 B. As shown in  FIG.  2    and as discussed above, each of the right optical assembly  103 A and the left optical subassembly  103 B may include the electronic display  104 , the movable first lens  112 , and the fixed second lens  114 . The first lens  112  and the second lens  114  of each of the optical assemblies  103  may form an optical lens pair  110 . The first lens  112  of each of the optical assemblies  103  may be movable in response to actuation of an actuator  128 . For example, the actuator  128  may be a stepper motor, a brushless DC motor, a voice-coil actuator (“VCA”), a piezoelectric actuator, a linear actuator, etc. A base  130  of the actuator  128  may be fixedly coupled to the frame  106  ( FIG.  1   ), and an output shaft  132  of the actuator  128  may be coupled to the first lens  112 , such as via the bracket  125 . Alternatively, the orientation of the actuator  128  may be reversed, with the base  130  coupled to the first lens  112  and the output shaft  132  coupled to the frame  106 . The actuator  128  may be oriented such that movement of the output shaft  132  occurs substantially parallel to the first optical axis  116  of the first lens  112 , resulting in movement of the first lens  112  substantially along the first optical axis  116 . 
     The movable first lens  112  is illustrated in  FIG.  2    as being positioned between (e.g., directly between) the electronic display  104  and the fixed second lens  114 . In additional embodiments, the movable first lens  112  may be positioned in place of the second lens  114 , and the second lens  114  may be positioned in place of the first lens  112 . In such embodiments, the fixed second lens  114  may be positioned between the electronic display  104  and the movable first lens  112 . 
       FIG.  3    illustrates a schematic side view of an optical assembly  300 , with optical rays  334  shown to demonstrate the movement of light from the electronic display  304  through an optical lens pair  310  including a movable first lens  312  and a fixed second lens  314 . In some examples, the optical assembly  300  may represent one or both of the optical assemblies  103  of  FIGS.  1  and  2   . The optical rays  334  may converge at a focal plane  336 , which may be configured to be at a user&#39;s eye (e.g., at a user&#39;s pupil) for viewing an image generated by the electronic display  304 . As the first lens  312  is moved toward or away from the second lens  314  as indicated by arrows  338 , such as by activation of the actuator  128  ( FIG.  2   ), a focal point (e.g., a location of the focal plane  336 ) of the optical assembly  300  may be adjusted. Accordingly, movement of the first lens  312  may result in a change of at least one optical property (e.g., focus) of the optical assembly  300 . 
       FIG.  4    is a perspective view of an optical assembly  400  of an HMD system including a flexure assembly  420 .  FIG.  5    is a detailed perspective view of the flexure assembly  420  of the HMD system. The optical assembly  400  may represent one or both of the optical assemblies  103  of  FIGS.  1  and  2   . As shown in  FIGS.  4  and  5   , the flexure assembly  420  may include a first flexure pair  440  and a second flexure pair  442 , which may be separated from each other by a gap  444 . The first flexure pair  440  may include a first flexure element  446  adjacent to a second flexure element  448 . The second flexure pair  442  may include a third flexure element  450  adjacent to a fourth flexure element  452 . A movable first end portion  454  (e.g., the upper end portion in the view of  FIGS.  4  and  5   ) of the flexure assembly  420  may be coupled to a movable first lens  412  (shown in dashed lines in  FIG.  4   ) of the optical assembly  400 , such as via a bracket  425 . A fixed second end portion  456  (e.g., the lower end portion in the view of  FIGS.  4  and  5   ) of the flexure assembly  420  may be coupled to a frame  406  of the HMD system. 
     The flexure elements  446 ,  448 ,  450 ,  452  may be configured to facilitate bending of the flexure assembly  420  in a desired direction (e.g., about an axis parallel to bending axis  458 , which extends parallel to and across a width of a major plane of the flexure elements  446 ,  448 ,  450 ,  452 ) while providing relative stiffness in a torsional direction (e.g., about an axis extending along a length of the flexure elements  446 ,  448 ,  450 ,  452 ). This configuration may result in movement of the first lens  412  substantially along an optical axis  416  thereof upon actuation of an actuator  428 . 
     The actuator  428  may be positioned and configured to move the first end portion  454  of the flexure assembly  420  and the first lens  412  relative to the frame  406 . For example, a base  430  of the actuator  428  may be coupled to the frame  406  and an output shaft  432  of the actuator  428  may be coupled to the bracket  425 , which in turn may be coupled to the first lens  412 . 
     As shown in  FIG.  5   , each of the flexure elements  446 ,  448 ,  450 ,  452  may include an enlarged central region  460  that has greater stiffness than end regions  462  thereof. Each of the flexure elements  446 ,  448 ,  450 ,  452  may include a base member  464  and a stiffener material  466  on the base member  464  in the central region  460 . In some embodiments, the stiffener material  466  and the base member  464  may be a unitary, integral material, such as may be formed by molding, extruding, or machining a material to form the flexure elements  446 ,  448 ,  450 ,  452 . In additional embodiments, the stiffener material  466  may be coupled to the base member  464 , such as by an adhesive, by overmolding the stiffener material  466  on the base member  464 , by fastening the stiffener material  466  to the base member  464 , by welding the stiffener material  466  to the base member  464 , by brazing the stiffener material  466  to the base member  464 , or a combination thereof. The stiffener material  466  may be positioned on one side of the base member  464 , on opposing sides of the base member  464 , or surrounding the base member  464  in the central region  460 . 
     In some embodiments, the base member  464  may be or include, for example, a metal material. For example, the metal material may be a titanium material or a steel material (e.g., a stainless-steel material). The stiffener material  466  may be or include, for example, a metal material, a polymer material, or a ceramic material. By way of example, the stiffener material  466  may be an epoxy material. The base material  464  may have a thickness of about 0.003 inch or less, such as about 0.002 inch or about 0.001 inch. The thickness of the base material  464  may affect a force required to move the first lens  412  and a mechanical stress experienced by the base material  464  when the first lens  412  is moved. A width of the base material  464  may also affect the force required to move the first lens  412 . In some examples, the actuation force for moving the first lens  412  a distance of about 2 mm may be less than 0.1 N. 
     The central region  460  of each of the flexure elements  446 ,  448 ,  450 ,  452  may have a greater thickness than the base member  464 , such as about 0.003 inch or more, such as about 0.004 inch, about 0.005 inch, or about 0.006 inch. The greater thickness of the central region  460  may result in the central region  460  having a greater stiffness than the end regions  462 , which may lack the stiffener material  466 . Thus, the end regions  462  may form living hinges, which may be regions where the base material  464  bends upon activation of the actuator  428  ( FIG.  4   ). 
     The gap  444  may be maintained by a first end block  468  and a second end block  470 . The first end block  468  may be coupled to the first end portion  454  of each of the flexure elements  446 ,  448 ,  450 ,  452 . The second end block  470  may be coupled to the second end portion  456  of each of the flexure elements  446 ,  448 ,  450 ,  452 . The first end block  468  and the second end block  470  may have a length defining the gap  444 . The presence of the gap  444  and the respective widths of the flexure elements  446 ,  448 ,  450 ,  452  may strengthen the flexure assembly  420  against torsional bending (e.g., bending about an axis parallel to a length of the flexure elements  446 ,  448 ,  450 ,  452 ) when the first lens  412  is moved by activation of the actuator  428 . 
     In some examples, an inner side of the flexure elements  446 ,  448 ,  450 ,  452  may include a concave cutout  472 , such as to provide space for other elements of the optical assembly  400 , such as the first lens  412  and/or the second lens  414 . In some embodiments, an outer side of the flexure elements  446 ,  448 ,  450 ,  452  may also be curved (e.g., may be convex). Each of the flexure elements  446 ,  448 ,  450 ,  452  may be substantially planar when in a state of rest (e.g., when the actuator  428  has not forced the flexure elements  446 ,  448 ,  450 ,  452  into a bent state). 
       FIG.  6    is a perspective view of a lens movement mechanism  600  including a flexure assembly  620 . As shown in  FIG.  6   , the flexure assembly  620  may include a movable first end portion  654  that is coupled to a movable optical lens  612  via a bracket  625 . The bracket  625  may be shaped and sized for coupling to an output shaft of an actuator, such as a VCA, linear actuator, brushless DC motor, stepper motor, piezoelectric actuator, etc. A force  674  may be applied by the actuator to the bracket  625  to bend the flexure assembly  620  and to move the optical lens  612  in a desired direction (e.g., along an optical axis  616  of the optical lens  612 ). A fixed second end portion  656  of the flexure assembly may be coupled to a frame of an HMD system. 
     The flexure assembly  620  may include first group  640  of two or more flexure elements  622  (at an upper side of the flexure assembly  620  in the view of  FIG.  6   ) and second group  642  of two or more flexure elements  622  (at a lower side of the flexure assembly  620  in the view of  FIG.  6   ). As described above, the flexure elements  622  may each include a base member  664  and an enlarged central region  660  with a stiffener material  666 . The first group  640  of flexure elements  622  may be separated from the second group  642  of flexure elements  622  by a gap  644 , which may increase a torsional stiffness of the flexure assembly  620  while allowing bending in a desired direction. The gap  644  may be defined and maintained by a first end block  668  and a second end block  670 . The flexure assembly  620  may guide the movement of the optical lens  612  by constraining movement of the optical lens  612  to movement substantially in the desired direction (e.g., along the optical axis  616  of the optical lens  612 ). 
       FIG.  7    is a perspective view of a lens movement mechanism  700  including a flexure assembly  720 , according to at least one additional embodiment of the present disclosure. The flexure assembly  720  may be configured and positioned to guide movement of an optical lens  712 , such as substantially along an optical axis  716  thereof. In this example, the flexure assembly  720  may include flexure elements  722  in a split configuration and an intermediate block  776 . A first movable end portion  778  of the flexure assembly  720  is illustrated at a back-left portion of the flexure assembly  720  in  FIG.  7   . The first movable end portion  778  of the flexible assembly  720  may be coupled to the optical lens  712 , such as via a first end block  780  and a bracket  725 . The bracket  725  may be positioned and configured for application of a force  774  by an actuator to move the optical lens  712 . A first flexure set  782  may extend from the first movable end portion  778  (e.g., from the first end block  780 ) to the intermediate block  776 . The intermediate block  776  is illustrated at a lower right portion of the flexure assembly  720  in the view of  FIG.  7   . The first flexure set  782  is illustrated at a back of the flexure assembly  720  in  FIG.  7   . A second fixed end portion  784  of the flexure assembly  720  may be coupled to a frame of an HMD system, such as via a second end block  786 . The second fixed end portion  784  of the flexure assembly  720  is illustrated at a front left portion of the flexure assembly  720  in  FIG.  7   . A second flexure set  788  may extend between the intermediate block  776  and the second fixed end portion  784  (e.g., the second end block  786 ) of the flexure assembly  720 . The second flexure set  788  is illustrated at a front portion of the flexure assembly in the view of  FIG.  7   . 
     As shown in  FIG.  7   , the first and second flexure sets  782 ,  788  may each include two or more (e.g., three, four, etc.) front flexure elements  790  and two or more (e.g., three, four, etc.) rear flexure elements  792 . The front flexure elements  790  may be flexure elements  722  positioned toward a front of the lens movement mechanism  700 , shown at an upper part of the flexure assembly  720  in  FIG.  7   . The rear flexure elements  792  may be flexure elements  722  positioned toward a rear of the lens movement mechanism  700 , shown at a lower part of the flexure assembly  720  in  FIG.  7   . The front flexure elements  790  may be separated from the rear flexure elements  792  by a gap  744 . In embodiments in which there are four front flexure elements  790  and four rear flexure elements  792  in each of the flexure sets  782 ,  788 , as illustrated in  FIG.  7   , these flexure sets  782 ,  788  may exhibit a substantially similar stiffness in a desired bending direction as comparable flexure sets with two front flexure elements and two rear flexure elements that may have the same thickness and material properties but twice the width (e.g., like the flexure assembly  620  of  FIG.  6   ). 
     The first flexure set  782  may be separated from the second flexure set  788  by a slit  794 , resulting in the split configuration. The split configuration of the flexure assembly  720  of  FIG.  7    may enable increased movement of the optical lens  712  in a desired direction while shortening a length of the flexure assembly  720 . This may enable the flexure assembly  720  to fit in a smaller space compared to a non-split configuration. In addition, the split configuration may improve a linearity of movement of the optical lens  712 . For example, moving an optical lens with a flexure assembly having a non-split configuration may result in cross-axis translation (e.g., radial translation) relative to the optical axis  716  (e.g., straightness error/decenter through travel) in the form of a parasitic arc-like path. This straightness error may be substantially predictable and may be accounted for and counteracted by software and/or hardware adjustments. On the other hand, moving an optical lens with the flexure assembly  720  having a split configuration like that shown in  FIG.  7    may mechanically result in substantially linear movement of the optical lens  712  along the optical axis  716 . For example, application of the force  774  on the bracket  725  may result in movement of the first end block  780  and bending of the first flexure set  782 . The intermediate block  776  may also move, resulting in bending of the second flexure set  788 . Any cross-axis translation of the intermediate block  776  due to bending of the second flexure set  788  may be counteracted by a substantially equal and opposite cross-axis translation of the first movable end portion  778  of the flexure assembly  720  due to bending of the first flexure set  782 . 
     A bending stiffness and/or a torsional stiffness of the flexure assembly  720  (or of any of the other flexure assemblies described herein) may be altered by adjusting one or more of the following example parameters: a thickness of the flexure element base members, a thickness of the flexure element stiffener materials, a length of the stiffener materials applied to the base members, a width of the flexure elements, a contour of the flexure elements, a length of the flexure elements, a quantity of flexure elements in each flexure set, a distance between the front flexure elements and the rear flexure elements (e.g., a length of the gap between the front flexure elements and the rear flexure elements), a material selection and/or material properties of the base members of the flexure elements, a material selection and/or material properties of the stiffener material of the flexure elements, etc. 
       FIG.  8    is a perspective view of a lens movement mechanism  800  including a flexure assembly  820 , according to at least one further embodiment of the present disclosure. The flexure assembly  820  may be configured and positioned to guide movement of an optical lens  812 , such as substantially along an optical axis  816  thereof. In this example, the flexure assembly  820  may include a first flexure subassembly  820 A including split flexure elements  822  and an intermediate block  876  and a second flexure subassembly  820 B including split flexure elements  822  and an intermediate block. The first flexure subassembly  820 A and the second flexure subassembly  820 B may be positioned on symmetrically opposite sides of the optical lens  812  (e.g., symmetrically about a centerline  813  of the optical lens  812  passing through the optical axis  816 ). Each of the flexure subassemblies  820 A,  820 B of the lens movement mechanism  800  of  FIG.  8    may be similar to the flexure assembly  720  of the lens movement mechanism  700  of  FIG.  7   . 
     In the example illustrated in  FIG.  8   , each flexure subassembly  820 A,  820 B is shown with two front split flexure elements  890  and two rear split flexure elements  892 . The distance between the front split flexure elements  890  and the rear split flexure elements  892  (e.g., a length of a gap  844  between the front split flexure elements  890  and the rear split flexure elements  892 ) may be smaller in the embodiment shown in  FIG.  8    compared to the embodiment shown in  FIG.  7   . The presence of the two flexure subassemblies  820 A,  820 B in the lens movement mechanism  800  of  FIG.  8    may enable the gap  844  between the front split flexure elements  890  and the rear split flexure elements  892  to be smaller, while still providing a torsional stiffness that is substantially the same as provided by the flexure assembly of  FIG.  7   . In addition, the symmetrical arrangement of the two flexure subassemblies  820 A,  820 B may further inhibit (e.g., reduce or eliminate) any off-axis movement (e.g., torsional twisting, cross-axis translation, etc.) of the optical lens  812  when an actuation force  874  is applied to the optical lens  812 , such as via a bracket  825 . 
       FIG.  9    is a perspective view of a lens movement mechanism  900  including a flexure assembly  920 , according to at least one other embodiment of the present disclosure. The flexure assembly  920  may be configured and positioned to guide movement of an optical lens  912 , such as substantially along an optical axis  916  thereof. In this example, the flexure assembly  920  may include a first flexure subassembly  920 A positioned on one side of the optical lens  912  and a second flexure subassembly  920 B positioned on an opposite side of the optical lens  912 . Each of the flexure subassemblies  920 A,  920 B may include a front flexure element  990  and a rear flexure element  992  separated by a gap  944 . In some embodiments, the front flexure element  990  and the rear flexure element  992  of the flexure subassemblies  920 A,  920 B may each include a concave cutout  972 , such as to accommodate the optical lens  912  and to avoid obstructing a view through the optical lens  912 . 
     A first movable end portion  978  of each flexure subassembly  920 A,  920 B may be coupled to the optical lens  912 , such as via a bracket  925 . A second fixed end portion  984  each flexure subassembly  920 A,  920   b  may be coupled to a frame of an HMD system. The first movable end portion  978  may be positioned proximate a first side of the optical lens  912 , and the second fixed end portion  984  may be positioned proximate a second, opposite side of the optical lens  912 . As illustrated in  FIG.  9   , respective lengths of the first and second flexure subassemblies  920 A,  920 B may be substantially parallel to each other. In other words, the first and second flexure subassemblies  920 A,  920 B may be substantially symmetrically located relative to the optical lens  912 . The symmetry of the configuration shown in  FIG.  9    may improve an off-axis stiffness of the lens movement mechanism  900  compared to other non-symmetrical configurations. 
       FIG.  10    is a perspective view of a lens movement mechanism  1000  including bearing elements, according to at least one embodiment of the present disclosure.  FIGS.  11  and  12    are detailed perspective views of portions of the lens movement mechanism  1000  of  FIG.  10   . The lens movement mechanism  1000  may be configured to guide movement of a movable optical lens  1012  relative to a fixed optical lens  1014 , an electronic display  1004 , and a frame  1006  of an HMD system supporting the fixed optical lens  1014  and the electronic display  1004 . As shown in  FIGS.  10 - 12   , the lens movement mechanism  1000  may include a bracket assembly  1025  coupled to the movable optical lens  1012 . The bracket assembly  1025  may include a fixed bracket portion  1002  and a movable bracket portion  1008 . The fixed bracket portion  1002  may be positioned between (e.g., sandwiched between) an outer movable bracket portion  1008 A and an inner movable bracket portion  1008 B of the movable bracket  1008 . The fixed bracket portion  1002  is illustrated in  FIG.  11   . The outer movable bracket portion  1008 A is illustrated in  FIG.  10    and the inner movable bracket portion  1008 B is illustrated in  FIG.  12   . 
     The bracket assembly  1025  may include one or more outer bearing guides  1018  ( FIG.  11   ) and one or more inner bearing guides  1020  ( FIG.  12   ). For example, the bearing guides  1018 ,  1020  may include V-shaped grooves, formed sheet metal grooves, sheet metal inserts, dowel pins, ceramic plate inserts, ceramic rod inserts, needle roller bearings (which may include, for example, ceramic and/or metal), etc., which may each act as a guide for respective bearing elements  1022 . Three outer bearing guides  1018  and three inner bearing guides  1020  are respectively illustrated in  FIGS.  11  and  12   , although other numbers of bearing guides  1018 ,  1020  may be included in different embodiments. As shown in  FIG.  11   , the outer bearing guides  1018  may be formed in a surface of the fixed bracket portion  1002 , with corresponding outer bearing guides  1018  also formed in a surface of the outer movable bracket portion  1008 A. As shown in  FIG.  12   , the inner bearing guides  1020  may be formed in a surface of the inner movable bracket portion  1008 B, with corresponding inner bearing guides  1020  also formed in a surface of the fixed bracket portion  1002 . The bearing elements  1022  may be positioned in the outer bearing guides  1018  and in the inner bearing guides  1022 . The bearing elements  1022  may be, for example, ball bearings or roller bearings. In some embodiments, a lubricant (e.g., grease) may also be applied to the outer bearing guides  1018  and inner bearing guides  1020 . 
     The outer movable bracket portion  1008 A may be coupled to the inner movable bracket portion  1008 B, such as via bolts  1024 . Flexible washers  1026  (e.g., O-rings) may be positioned around the bolts  1024 , such as to apply a preload (e.g., a compressive force) between the movable bracket portion  1008  and the fixed bracket portion  1002  and to maintain the bearing elements  1022  within the respective bearing guides  1018 ,  1020 . The preload may also provide a stiffness to the lens movement mechanism  700  (e.g., a resistance to off-axis motion of the movable optical lens  1012 ). As illustrated in  FIG.  11   , the fixed bracket portion  1002  may include cutouts  1034  through which the bolts  1024  may pass. The cutouts  1034  may be larger than the bolts  1024  and the flexible washers  1026 , such that the bolts  1024  and flexible washers  1026  may move back and forth within the cutouts  1034 . 
     An actuator  1028  (e.g., a VCA, linear actuator, brushless DC motor, stepper motor, piezoelectric actuator, etc.) may be positioned and configured to move the movable bracket portion  1008  and movable optical lens  1012  relative to the fixed bracket portion  1002  and to the frame  1006  of the HMD system. For example, a base  1030  of the actuator  1028  may be coupled to the frame  1006  and an output shaft  1032  of the actuator  1028  may be coupled to the movable bracket portion  1008 . Alternatively, the base  1030  of the actuator  1028  may be coupled to the movable bracket portion  1008  and the output shaft  1032  of the actuator  1028  may be coupled to the frame  1006 . The movable bracket portion  1008  may be configured to translate (e.g., slide, roll, etc.) in a desired direction (e.g., substantially along an optical axis  1016  of the movable optical lens  1012 ) to move the movable optical lens  1012  relative to the frame  1006  of the HMD system. The bearing elements  1022  and corresponding bearing guides  1018 ,  1020  may guide (e.g., constrain) the movement of the movable optical lens  1012  in the desired direction. 
       FIG.  13    is a perspective view of a lens movement mechanism  1300  that includes a bearing assembly  1302 .  FIG.  14    is a perspective view of one example configuration of the bearing assembly  1302 A and  FIG.  15    is a cross-sectional view of another example configuration of the bearing assembly  1302 B. As shown in  FIG.  13   , the lens movement mechanism  1300  may include an actuator  1304  (e.g., a VCA, linear actuator, brushless DC motor, stepper motor, piezoelectric actuator, etc.) positioned and configured to move a movable optical lens  1306  in a desired direction (e.g., along an optical axis of the movable optical lens  1306 ). The bearing assembly  1302  may be configured to guide movement of the movable optical lens  1306  relative to a fixed optical lens  1308 , an electronic display  1310 , and a frame  1312  of an HMD system supporting the fixed optical lens  1308  and the electronic display  1310 . The bearing assembly  1302  may include a bracket assembly  1314  including a fixed bracket portion  1316  coupled to the frame  1312  and a movable bracket portion  1318  coupled to the movable optical lens  1306 . Bearing elements  1320  (shown in  FIGS.  14  and  15   ) may be positioned between the fixed bracket portion  1316  and the movable bracket portion  1318 . The bearing elements  1320  may be configured to roll and/or slide between the fixed bracket portion  1316  and the movable bracket portion  1318 . Bearing guides  1319  (e.g., pins, rods, etc.) may also be positioned between the fixed bracket portion  1316  and the movable bracket portion  1318 , and the bearing elements  1320  may be configured to roll and/or slide against the bearing guides  1319 . The movable bracket portion  1318  and the movable optical lens  1306  may be configured to move along the fixed bracket portion  1316  upon activation of the actuator  1304 . 
     As shown in  FIGS.  13  and  14   , a preload may be applied to the bearing elements  1320  by a preload tab  1322  and corresponding bolt  1324 . For example, a portion of the movable bracket portion  1318  may be held in place relative to the remainder of the movable bracket portion  1318  by the preload tab  1322 , and the bolt  1324  may be tightened until a predetermined preload is applied to the bearing elements  1320 . By way of additional examples, a preload may be applied to the bearing elements  1320  via angled fasteners, metal coil and sheet springs, spring pins, elastomeric springs or bands, fixture elements including a transducer (e.g., a load cell) and a micrometer positioning stage (e.g., a movable stage configured to position a load cell to apply a preload force to the bearing elements  1320 ), and/or set screws, etc. 
     As illustrated in  FIGS.  14  and  15   , one or more of the bearing elements  1320  may, in some embodiments, include a toothed gear  1326 . In the example shown in  FIG.  14   , a center one of the bearing elements  1320  may include a toothed gear  1326 . In the example shown in  FIG.  15   , each of the bearing elements  1320  may include a toothed gear  1326 . A complementary rack  1328  may be coupled to the fixed bracket portion  1316  and/or to the movable bracket portion  1318 . When the movable bracket portion  1318  moves along the fixed bracket portion  1316 , the gears of the bearing elements  1320  may be engaged with and roll along the rack(s)  1326 . Configurations with toothed gears  1326  and complementary racks  1328  may reduce creep in the bearing elements  1320  and improve the function and/or life of the lens movement mechanism  1300  compared to some other bearing element types by preventing bearing creep. For example, the toothed gears  1326  and racks  1328  may cause the bearing elements  1320  to maintain their relative position between the fixed bearing guide and the moving bearing guide. Without the toothed gears  1326  and racks  1328 , the bearing elements  1320  may exhibit creep (e.g., the bearing elements  1320  slipping and migrating toward one end of the guideway) due to unbalanced forces from friction, preload, inertia, gravity, etc. As the bearing elements  1320  move further from their initial position, the bearing elements  1320  may prematurely contact end stops, which may result in the mechanism jamming. Consequently, travel of the mechanism may be reduced and/or friction may significantly increase as the bearing elements  1320  are dragged by one bearing guide and forced to slide on the other until both end stops are contacted. 
     As illustrated in  FIG.  14   , in some embodiments the bearing assembly  1302 A may include a bearing cage  1330  that may be configured to maintain the bearing elements  1320  at a predetermined distance from each other, such as to keep the bearing elements  1320  from creeping relative to one another. This may prevent the bearing elements  1320  from contacting one another, which would otherwise increase friction and/or wear. 
     Although  FIGS.  14  and  15    illustrate three bearing elements  1320  in each set of bearing elements  1320 , the present disclosure is not so limited. For example, the bearing assembly  1302  may include one or more sets of bearing elements  1320  that each include two, three, four, five, or more bearing elements  1320 . 
       FIG.  16    is a detailed perspective view of a lens movement mechanism  1600 , according to at least one additional embodiment of the present disclosure. The lens movement mechanism  1600  may be similar to the lens movement mechanism  1300  described above in some respects. For example, the lens movement mechanism  1600  may include a bearing assembly  1602  and an actuator  1604  positioned and configured to move a movable optical lens  1606  in a desired direction. The bearing assembly  1602  may be configured to move the movable optical lens  1606  relative to a fixed optical lens  1608 , an electronic display  1610 , and a frame  1612  of an HMD system. Bearing elements  1620  (two of which are shown in  FIG.  16    in dashed lines) may be positioned to roll and/or slide between relatively movable portions of the bearing assembly  1602 , such as bearing guides  1619 . 
     As illustrated in  FIG.  16   , the bearing assembly  1602  may be positioned to at least partially surround the actuator  1604 . For example, a first bearing set may be positioned on one side of the actuator  1604  and a second bearing set may be positioned on an opposite side of the actuator  1604 . Set screws  1632  may be positioned to apply a preload to the bearing elements  1620 , such as by applying a pressure to the bearing guides  1619 . 
       FIG.  17    is a partially cut-away perspective view of a lens movement mechanism  1700 , according to at least one further embodiment of the present disclosure. The lens movement mechanism  1700  may be similar to the lens movement mechanism  1300  described above in some respects. For example, the lens movement mechanism  1700  may include a bearing assembly  1702  and an actuator  1704  positioned and configured to move a movable optical lens in a desired direction. The bearing assembly  1702  may be configured to move the movable optical lens relative to a fixed optical lens, an electronic display, and a frame  1712  of an HMD system. The bearing assembly  1702  may include a fixed bracket portion  1716  and a movable bracket portion  1718 . Bearing elements may be positioned to roll and/or slide between the fixed bracket portion  1716  and the movable bracket portion  1718 . 
     As shown in  FIG.  17   , set screws  1732  may be positioned to apply a preload to the bearing elements of the bearing assembly  1702 , such as by applying a pressure to bearing guides. A sensor element  1734  (e.g., a permanent magnet) may be mounted to the movable bracket portion  1718  to facilitate determining a position of the movable bracket portion  1718  relative to the frame  1712 . One or more assembly plates  1736  may be positioned along a lateral side (from the perspective of  FIG.  17   ) of the bearing assembly  1702  to hold the components of the bearing assembly  1702  in place. 
       FIG.  18    is a detailed perspective view of a lens movement mechanism  1800 , according to at least one other embodiment of the present disclosure. The lens movement mechanism  1800  may be similar to the lens movement mechanism  1300  described above in some respects. For example, the lens movement mechanism  1800  may include a bearing assembly  1802  and an actuator  1804  positioned and configured to move a movable optical lens  1806  in a desired direction. The bearing assembly  1802  may be configured to move the movable optical lens  1806  relative to a fixed optical lens  1808 , an electronic display, and a frame  1812  of an HMD system. The bearing assembly  1802  may include a fixed bracket portion  1816  and a movable bracket portion  1818 . Bearing elements  1820  may be positioned to roll and/or slide between the fixed bracket portion  1816  and the movable bracket portion  1818 . 
     As shown in  FIG.  18   , set screws  1832  may be positioned to apply a preload to the bearing elements  1820  of the bearing assembly  1802 , such as by applying a pressure to bearing guides  1819 . A sensor element  1834  (e.g., a permanent magnet) may be mounted to the movable bracket portion  1818  to facilitate determining a position of the movable bracket portion  1818  relative to the frame  1812 . One or more assembly plates  1836  may be positioned at a top side (from the perspective of  FIG.  18   ) of the bearing assembly  1802  to hold the components of the bearing assembly  1802  in place. 
       FIG.  19    is a partially cut-away perspective view of a lens movement mechanism  1900  including a lens movement stop mechanism  1902  (e.g., a brake mechanism), according to at least one embodiment of the present disclosure.  FIG.  20    is front view of the lens movement mechanism  1900  and the lens movement stop mechanism  1902  of  FIG.  19   .  FIG.  21    is a detailed perspective view of the lens movement stop mechanism  1902  of  FIG.  19   .  FIG.  22    is a detailed front view of the lens movement stop mechanism  1902  of  FIG.  19   .  FIG.  23    is another detailed perspective view of the lens movement stop mechanism  1902  of  FIG.  19   . As shown in  FIGS.  19 - 23   , the lens movement mechanism  1900  may include a flexure assembly  1920 , a bracket  1925  coupling the flexure assembly  1920  to a movable optical lens  1912 , and an actuator  1928  (e.g., a VCA, linear actuator, brushless DC motor, stepper motor, piezoelectric actuator, etc.) positioned and configured to move the bracket  1925  and the movable optical lens  1912  in a desired direction. The movable optical lens  1912  may be movable relative to an electronic display  1904 , a frame  1906  of a corresponding HMD system, and a fixed optical lens  1914 . Some types of actuators  1928  for moving the movable optical lens  1912 , such as VCAs, may not inherently stop movement of the movable optical lens  1912  when the actuator  1928  is powered down. For example, an output shaft  1932  of the actuator  1928  may substantially freely axially move when the actuator  1928  is powered down. In embodiments of the present disclosure, this may result in the movable optical lens  1912  substantially freely moving, such as due to gravity, inertia from head-movement, or other forces. 
     Thus, in some examples, the lens movement mechanism  1900  may include the lens movement stop mechanism  1902 , which may include a protrusion  1908  (e.g., a fin) extending from the bracket  1925  and a clamp  1910  positioned and configured to abut against the protrusion  1908  to stop movement of the bracket  1925  and movable optical lens  1912 . For example, the clamp  1910  may include a brake element  1916  (e.g., a polymer material such as an O-ring material, an elastomer, or another brake material) positioned to abut against the protrusion  1908  when the lens movement stop mechanism  1902  is actuated. In some embodiments, the protrusion  1908  may include one or more slots or holes and the brake element  1916  may include one or more corresponding pins or bumps. Clamp arms  1918  of the clamp  1910  may rotate about a pivot pin  1922 . In some embodiments, the pivot pin  1922  may define a common axis of rotation of the clamp arms  1918 . This axis of rotation may, in some examples, be substantially parallel to an optical axis of the movable optical lens  1912 . Ends of the clamp arms  1918  opposite the brake element  1916  may be operably coupled to a brake actuator  1924 . Upon activation of the brake actuator  1924 , the clamp arms  1918  may be rotated and the brake element  1916  may abut against the protrusion  1908  of the bracket  1925  to stop movement of the bracket  1925  and movable optical lens  1912 . 
     Although the lens movement stop mechanism  1902  of  FIGS.  19 - 23    is illustrated with the clamp  1910 , the present disclosure is not so limited. For example, other brake mechanism types may be employed, such as spring-biased pins, linear actuators, leadscrew actuators, etc. 
     In some examples, the brake actuator  1924  used to activate the lens movement stop mechanism  1902  may include a wire  1926  that may be or include a shape-memory alloy (“SMA”), such as nitinol. As shown in  FIGS.  19 - 23   , the SMA wire  1926  may be wrapped around the clamp arms  1918  such that when the SMA wire  1926  is lengthened (e.g., upon removal of an applied heat and/or electrical current in the SMA wire  1926 ), a spring element  1930  (e.g., a torsion spring) (illustrated in  FIG.  7   ) may bias the clamp arms  1918  to apply a sufficient pressure on the protrusion  1908  with the brake elements  1916  to inhibit (e.g., reduce or eliminate) movement of the bracket  1925  and, consequently, of the movable optical lens  1912 . When the SMA wire  1926  is constricted (e.g., upon application sufficient heat and/or electrical current in the SMA wire  1926 ), the clamp arms  1918  may overcome a force applied by the spring element  1930  to release sufficient pressure on the brake elements  1916  to allow the bracket  1925  and movable optical lens  1912  to move in a desired direction. 
     In some examples, heat (e.g., resistive heat) may be applied to the SMA wire  1926  by application of a sufficient voltage to electrodes  1934  coupled to ends of the SMA wire  1926 . In additional examples, waste heat from electrical circuitry used to operate a corresponding HMD may be used to heat the SMA wire  1926 . In any case, a powered-down state of the HMD may result in the lens movement stop mechanism  1902  stopping movement of the movable optical lens  1912 . 
     Although the lens movement stop mechanism  1902  is shown in  FIGS.  19 - 23    as including an SMA wire  1926  as part of the brake actuator  1924 , other types of actuators (e.g., stepper motors, linear actuators, etc.) are also options for the brake actuator  1924 . 
       FIG.  24    is a perspective view of an optical assembly  2400  including shock protection elements  2402 .  FIG.  25    is a cross-sectional view of the optical assembly of  FIG.  24   . The shock protection elements  2402  may include a shock-absorbing material (e.g., an elastomeric material, a foam material, etc.). The shock protection elements  2402  may be positioned and configured to absorb a shock event, such as dropping, striking, or otherwise quickly moving the optical assembly  2400 , to reduce a shock felt by a movable optical lens  2404  of the optical assembly  2400 . For example, the shock protection elements  2402  may be positioned at one or more locations on a frame  2412  of the optical assembly  2400  such that the movable optical lens  2404  may contact the shock protection elements  2402  if the movable optical lens  2404  is forced to a maximum forward or backward position (e.g., forward or backward relative to an optical axis  2440  of the movable optical lens  2404 ). The reduced shock may reduce potential damage to the movable optical lens  2404 . The reduced shock may also improve a positional accuracy (e.g., alignment, etc.) of the movable optical lens  2404 . 
       FIG.  26 A  is a perspective view of an optical assembly  2600  including a shock protection element  2602 , according to at least one additional embodiment of the present disclosure.  FIG.  26 B  is a partial cross-sectional view of the optical assembly  2600  and shock protection element  2602 . In this example, the shock protection element  2602  may include a flexible enclosure holding a finite volume of fluid (e.g., gas or liquid), such as a molded rubber enclosure. The optical assembly  2600  may be mounted within a corresponding HMD system with elastomeric mounts (e.g., elastomeric washers, an outer surface of the shock protection element  2602 , etc.) to provide some potential relative movement between the optical assembly  2600  and the HMD system. Upon a shock event, the shock protection element  2602  may press against a frame of the HMD system, causing the shock protection element  2602  to deform. 
     As shown in  FIG.  26 B , the shock protection element  2602  may include one or more expansible features  2604 , such as to absorb and distribute shock energy. Like an automobile airbag, a shock event may result in these expansible features  2604  expanding to provide a cushion for a movable optical lens  2606  of the optical assembly  2600 . Although only one shock protection element  2602  is visible in the views of  FIGS.  26 A and  26 B , in some embodiments the optical assembly  2600  may include multiple (e.g., two, three, four, or more) shock protection elements  2602  to protect the optical assembly  2600  in case of shock events applying forces to the optical assembly  2600  from different directions. 
       FIG.  27    is a partial cross-sectional view of an optical assembly  2700  including a shock protection element  2702 , according to at least one further embodiment of the present disclosure. The shock protection element  2702  may be positioned and configured to provide a flexible stop against which a movable optical lens  2704  of the optical assembly  2700  may abut during a shock event. For example, the shock protection element  2702  may be a unitary, integral piece of material (e.g., polymer material, elastomer material, metal material, etc.) including a bumper surface  2706  and one or more inner arms  2708  positioned on opposing sides of the movable optical lens  2704 . If an inward force is applied to the bumper surface  2706  (e.g., during a shock event), the inner arms  2708  may flex in a direction parallel to the optical axis to limit movement of the movable optical lens  2704 . 
       FIG.  28    is a flow diagram illustrating a method  2800  of varying at least one optical property (e.g., a focal distance) of an optical lens system, according to at least one embodiment of the present disclosure. At operation  2810 , a first optical lens may be moved relative to a frame, such as a frame of an HMD. The first optical lens may be positioned between a second optical lens and an electronic display that are stationary relative to the frame. Operation  2810  may be performed in a variety of ways, such as any of the ways discussed above. For example, the first optical lens may be moved with an actuator, such as a VCA, linear actuator, brushless DC motor, stepper motor, piezoelectric actuator, etc. 
     At operation  2820 , the movement of the first optical lens may be guided with a flexure assembly. The flexure assembly may have a first movable end portion coupled to the first optical lens (e.g., via a bracket) and a second fixed end portion coupled to the frame element. Operation  2820  may be performed in a variety of ways, such as any of the ways discussed above. 
       FIG.  29    is a flow diagram illustrating a method  2900  of fabricating a display subassembly of an HMD, according to at least one embodiment of the present disclosure. At operation  2910 , an electronic display and a stationary optical lens may be fixedly coupled to a frame (e.g., a frame of an HMD). At operation  2920 , a movable optical lens may be positioned between the electronic display and the stationary optical lens. At operation  2930 , a first movable end portion of a flexure assembly may be coupled to the movable optical lens, and a second fixed end portion of the flexure assembly may be coupled to the frame. Operations  2910 ,  2920 , and  2930  may be performed in a variety of ways, such as any of the ways discussed above. The flexure assembly may be configured to guide movement of the movable optical lens in a desired direction, such as substantially along an optical axis of the movable optical lens. 
       FIG.  30    is a flow diagram illustrating a method  3000  of making varifocal adjustments, according to at least one embodiment of the present disclosure. At operation  3010 , a first lens may be moved relative to a frame (e.g., a frame of an HMD system) supporting the first lens and relative to a second lens fixedly coupled to the frame. The lens may be moved from a first position to a second position. Operation  3010  may be performed in a variety of ways, such as any of the ways discussed above. For example, the first lens may be moved by actuating a voice coil actuator that is coupled between the first lens (e.g., via a mounting bracket) and the frame. The movement of the first lens may be guided by a flexure assembly, as described above. 
     At operation  3020 , the first lens may be maintained in the second position by applying, with a brake mechanism, a braking pressure against a mounting bracket coupled to the first lens. Operation  3020  may be performed in a variety of ways, such as any of the ways discussed above. In some examples, a clamp may apply the braking pressure to a protrusion of the mounting bracket. For example, the clamp may be released by constriction of an SMA wire, and a spring element (e.g., a torsional spring) may apply the braking pressure after the SMA wire is lengthened. 
     Accordingly, the present disclosure includes systems, methods, and devices for making varifocal adjustments in a controlled and predictable manner. In some embodiments, flexure assemblies and/or ball-bearing assemblies may guide movement of a movable optical lens in a desired direction, such as substantially along an optical axis of the movable optical lens. Various example configurations of flexure assemblies and ball-bearing assemblies are described in the present disclosure. In addition, the present disclosure includes lens movement stop mechanisms, such as for use with actuators that have a freely moving output shaft when powered down (e.g., VCAs). The disclosed systems, methods, and devices provide improved configurations compared to conventional varifocal systems, such as by enabling compact and lightweight designs that may be suitable for use in HMD systems. 
     Embodiments of the present disclosure may include or be implemented in conjunction with various types of artificial-reality systems. Artificial reality is a form of reality that has been adjusted in some manner before presentation to a user, which may include, for example, a virtual reality, an augmented reality, a mixed reality, a hybrid reality, or some combination and/or derivative thereof. Artificial-reality content may include completely computer-generated content or computer-generated content combined with captured (e.g., real-world) content. The artificial-reality content may include video, audio, haptic feedback, or some combination thereof, any of which may be presented in a single channel or in multiple channels (such as stereo video that produces a 3D effect to the viewer). Additionally, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, for example, create content in an artificial reality and/or are otherwise used in (e.g., to perform activities in) an artificial reality. 
     Artificial-reality systems may be implemented in a variety of different form factors and configurations. Some artificial-reality systems may be designed to work without near-eye displays (NEDs). Other artificial-reality systems may include an NED that also provides visibility into the real world (such as, e.g., augmented-reality system  3100  in  FIG.  31   ) or that visually immerses a user in an artificial reality (such as, e.g., virtual-reality system  3200  in  FIG.  32   ). While some artificial-reality devices may be self-contained systems, other artificial-reality devices may communicate and/or coordinate with external devices to provide an artificial-reality experience to a user. Examples of such external devices include handheld controllers, mobile devices, desktop computers, devices worn by a user, devices worn by one or more other users, and/or any other suitable external system. 
     Turning to  FIG.  31   , the augmented-reality system  3100  may include an eyewear device  3102  with a frame  3110  configured to hold a left display device  3115 (A) and a right display device  3115 (B) in front of a user&#39;s eyes. The display devices  3115 (A) and  3115 (B) may act together or independently to present an image or series of images to a user. While the augmented-reality system  3100  includes two displays, embodiments of this disclosure may be implemented in augmented-reality systems with a single NED or more than two NEDs. 
     In some embodiments, the augmented-reality system  3100  may include one or more sensors, such as sensor  3140 . The sensor  3140  may generate measurement signals in response to motion of the augmented-reality system  3100  and may be located on substantially any portion of the frame  3110 . The sensor  3140  may represent one or more of a variety of different sensing mechanisms, such as a position sensor, an inertial measurement unit (IMU), a depth camera assembly, a structured light emitter and/or detector, or any combination thereof. In some embodiments, the augmented-reality system  3100  may or may not include the sensor  3140  or may include more than one sensor. In embodiments in which the sensor  3140  includes an IMU, the IMU may generate calibration data based on measurement signals from the sensor  3140 . Examples of the sensor  3140  may include, without limitation, accelerometers, gyroscopes, magnetometers, other suitable types of sensors that detect motion, sensors used for error correction of the IMU, or some combination thereof. 
     In some examples, the augmented-reality system  3100  may also include a microphone array with a plurality of acoustic transducers  3120 (A)- 3120 (J), referred to collectively as acoustic transducers  3120 . The acoustic transducers  3120  may represent transducers that detect air pressure variations induced by sound waves. Each acoustic transducer  3120  may be configured to detect sound and convert the detected sound into an electronic format (e.g., an analog or digital format). The microphone array in  FIG.  32    may include, for example, ten acoustic transducers: acoustic transducers  3120 (A) and  3120 (B), which may be designed to be placed inside a corresponding ear of the user, acoustic transducers  3120 (C),  3120 (D),  3120 (E),  3120 (F),  3120 (G), and  3120 (H), which may be positioned at various locations on frame  3110 , and/or acoustic transducers  3120 (I) and  3120 (J), which may be positioned on a corresponding neckband  3105 . 
     In some embodiments, one or more of the acoustic transducers  3120 (A)-(F) may be used as output transducers (e.g., speakers). For example, the acoustic transducers  3120 (A) and/or  3120 (B) may be earbuds or any other suitable type of headphone or speaker. 
     The configuration of the acoustic transducers  3120  of the microphone array may vary. While the augmented-reality system  3100  is shown in  FIG.  31    as having ten acoustic transducers  3120 , the number of acoustic transducers  3120  may be greater or less than ten. In some embodiments, using higher numbers of acoustic transducers  3120  may increase the amount of audio information collected and/or the sensitivity and accuracy of the audio information. In contrast, using a lower number of acoustic transducers  3120  may decrease the computing power required by an associated controller  3150  to process the collected audio information. In addition, the position of each acoustic transducer  3120  of the microphone array may vary. For example, the position of an acoustic transducer  3120  may include a defined position on the user, a defined coordinate on frame  3110 , an orientation associated with each acoustic transducer  3120 , or some combination thereof. 
     The acoustic transducers  3120 (A) and  3120 (B) may be positioned on different parts of the user&#39;s ear, such as behind the pinna, behind the tragus, and/or within the auricle or fossa. Or, there may be additional acoustic transducers  3120  on or surrounding the ear in addition to acoustic transducers  3120  inside the ear canal. Having an acoustic transducer  3120  positioned next to an ear canal of a user may enable the microphone array to collect information on how sounds arrive at the ear canal. By positioning at least two of the acoustic transducers  3120  on either side of a user&#39;s head (e.g., as binaural microphones), the augmented-reality device  3100  may simulate binaural hearing and capture a 3D stereo sound field around about a user&#39;s head. In some embodiments, the acoustic transducers  3120 (A) and  3120 (B) may be connected to the augmented-reality system  3100  via a wired connection  3130 , and in other embodiments the acoustic transducers  3120 (A) and  3120 (B) may be connected to augmented-reality system  3100  via a wireless connection (e.g., a Bluetooth connection). In still other embodiments, the acoustic transducers  3120 (A) and  3120 (B) may not be used at all in conjunction with the augmented-reality system  3100 . 
     The acoustic transducers  3120  on the frame  3110  may be positioned in a variety of different ways, including along the length of the temples, across the bridge, above or below the display devices  3115 (A) and  3115 (B), or some combination thereof. The acoustic transducers  3120  may also be oriented such that the microphone array is able to detect sounds in a wide range of directions surrounding the user wearing the augmented-reality system  3100 . In some embodiments, an optimization process may be performed during manufacturing of the augmented-reality system  3100  to determine relative positioning of each acoustic transducer  3120  in the microphone array. 
     In some examples, the augmented-reality system  3100  may include or be connected to an external device (e.g., a paired device), such as the neckband  3105 . The neckband  3105  generally represents any type or form of paired device. Thus, the following discussion of the neckband  3105  may also apply to various other paired devices, such as charging cases, smart watches, smart phones, wrist bands, other wearable devices, hand-held controllers, tablet computers, laptop computers, other external compute devices, etc. 
     As shown, the neckband  3105  may be coupled to the eyewear device  3102  via one or more connectors. The connectors may be wired or wireless and may include electrical and/or non-electrical (e.g., structural) components. In some cases, the eyewear device  3102  and the neckband  3105  may operate independently without any wired or wireless connection between them. While  FIG.  31    illustrates the components of the eyewear device  3102  and the neckband  3105  in example locations on the eyewear device  3102  and the neckband  3105 , the components may be located elsewhere and/or distributed differently on the eyewear device  3102  and/or the neckband  3105 . In some embodiments, the components of the eyewear device  3102  and the neckband  3105  may be located on one or more additional peripheral devices paired with the eyewear device  3102 , the neckband  3105 , or some combination thereof. 
     Pairing external devices, such as the neckband  3105 , with augmented-reality eyewear devices may enable the eyewear devices to achieve the form factor of a pair of glasses while still providing sufficient battery and computation power for expanded capabilities. Some or all of the battery power, computational resources, and/or additional features of the augmented-reality system  3100  may be provided by a paired device or shared between a paired device and an eyewear device, thus reducing the weight, heat profile, and form factor of the eyewear device overall while still retaining desired functionality. For example, the neckband  3105  may allow components that would otherwise be included on an eyewear device to be included in the neckband  3105  since users may tolerate a heavier weight load on their shoulders than they would tolerate on their heads. The neckband  3105  may also have a larger surface area over which to diffuse and disperse heat to the ambient environment. Thus, the neckband  3105  may allow for greater battery and computation capacity than might otherwise have been possible on a stand-alone eyewear device. Since weight carried in the neckband  3105  may be less invasive to a user than weight carried in the eyewear device  3102 , a user may tolerate wearing a lighter eyewear device and carrying or wearing the paired device for greater lengths of time than a user would tolerate wearing a heavy standalone eyewear device, thereby enabling users to more fully incorporate artificial-reality environments into their day-to-day activities. 
     The neckband  3105  may be communicatively coupled with the eyewear device  3102  and/or to other devices. These other devices may provide certain functions (e.g., tracking, localizing, depth mapping, processing, storage, etc.) to the augmented-reality system  3100 . In the embodiment of  FIG.  31   , the neckband  3105  may include two acoustic transducers (e.g.,  3120 (I) and  3120 (J)) that are part of the microphone array (or potentially form their own microphone subarray). The neckband  3105  may also include a controller  3125  and a power source  3135 . 
     The acoustic transducers  3120 (I) and  3120 (J) of the neckband  3105  may be configured to detect sound and convert the detected sound into an electronic format (analog or digital). In the embodiment of  FIG.  31   , the acoustic transducers  3120 (I) and  3120 (J) may be positioned on the neckband  3105 , thereby increasing the distance between the neckband acoustic transducers  3120 (I) and  3120 (J) and other acoustic transducers  3120  positioned on the eyewear device  3102 . In some cases, increasing the distance between the acoustic transducers  3120  of the microphone array may improve the accuracy of beamforming performed via the microphone array. For example, if a sound is detected by the acoustic transducers  3120 (C) and  3120 (D) and the distance between the acoustic transducers  3120 (C) and  3120 (D) is greater than, e.g., the distance between the acoustic transducers  3120 (D) and  3120 (E), the determined source location of the detected sound may be more accurate than if the sound had been detected by the acoustic transducers  3120 (D) and  3120 (E). 
     The controller  3125  of the neckband  3105  may process information generated by the sensors on the neckband  3105  and/or the augmented-reality system  3100 . For example, the controller  3125  may process information from the microphone array that describes sounds detected by the microphone array. For each detected sound, the controller  3125  may perform a direction-of-arrival (DOA) estimation to estimate a direction from which the detected sound arrived at the microphone array. As the microphone array detects sounds, the controller  3125  may populate an audio data set with the information. In embodiments in which augmented-reality system  3100  includes an inertial measurement unit, the controller  3125  may compute all inertial and spatial calculations from the IMU located on the eyewear device  3102 . A connector may convey information between the augmented-reality system  3100  and the neckband  3105  and between the augmented-reality system  3100  and the controller  3125 . The information may be in the form of optical data, electrical data, wireless data, or any other transmittable data form. Moving the processing of information generated by the augmented-reality system  3100  to the neckband  3105  may reduce weight and heat in the eyewear device  3102 , making it more comfortable to the user. 
     A power source  3135  in the neckband  3105  may provide power to the eyewear device  3102  and/or to the neckband  3105 . The power source  3135  may include, without limitation, lithium ion batteries, lithium-polymer batteries, primary lithium batteries, alkaline batteries, or any other form of power storage. In some cases, the power source  3135  may be a wired power source. Including the power source  3135  on the neckband  3105  instead of on the eyewear device  3102  may help better distribute the weight and heat generated by the power source  3135 . 
     As noted, some artificial-reality systems may, instead of blending an artificial reality with actual reality, substantially replace one or more of a user&#39;s sensory perceptions of the real world with a virtual experience. One example of this type of system is a head-worn display system, such as virtual-reality system  3200  in  FIG.  32   , that mostly or completely covers a user&#39;s field of view. The virtual-reality system  3200  may include a front rigid body  3202  and a band  3204  shaped to fit around a user&#39;s head. The virtual-reality system  3200  may also include output audio transducers  3206 (A) and  3206 (B). Furthermore, while not shown in  FIG.  32   , the front rigid body  3202  may include one or more electronic elements, including one or more electronic displays, one or more inertial measurement units (IMUs), one or more tracking emitters or detectors, and/or any other suitable device or system for creating an artificial-reality experience. 
     Artificial-reality systems may include a variety of types of visual feedback mechanisms. For example, display devices in the augmented-reality system  3100  and/or the virtual-reality system  3200  may include one or more liquid crystal displays (LCDs), light emitting diode (LED) displays, organic LED (OLED) displays, digital light project (DLP) micro-displays, liquid crystal on silicon (LCoS) micro-displays, and/or any other suitable type of display screen. These artificial-reality systems may include a single display screen for both eyes or may provide a display screen for each eye, which may allow for additional flexibility for varifocal adjustments or for correcting a user&#39;s refractive error. Some of these artificial-reality systems may also include optical subsystems having one or more lenses (e.g., conventional concave or convex lenses, Fresnel lenses, adjustable liquid lenses, etc.) through which a user may view a display screen. These optical subsystems may serve a variety of purposes, including to collimate (e.g., make an object appear at a greater distance than its physical distance), to magnify (e.g., make an object appear larger than its actual size), and/or to relay (to, e.g., the viewer&#39;s eyes) light. These optical subsystems may be used in a non-pupil-forming architecture (such as a single lens configuration that directly collimates light but results in so-called pincushion distortion) and/or a pupil-forming architecture (such as a multi-lens configuration that produces so-called barrel distortion to nullify pincushion distortion). 
     In addition to or instead of using display screens, some the artificial-reality systems described herein may include one or more projection systems. For example, display devices in the augmented-reality system  3100  and/or the virtual-reality system  3200  may include micro-LED projectors that project light (using, e.g., a waveguide) into display devices, such as clear combiner lenses that allow ambient light to pass through. The display devices may refract the projected light toward a user&#39;s pupil and may enable a user to simultaneously view both artificial-reality content and the real world. The display devices may accomplish this using any of a variety of different optical components, including waveguide components (e.g., holographic, planar, diffractive, polarized, and/or reflective waveguide elements), light-manipulation surfaces and elements (such as diffractive, reflective, and refractive elements and gratings), coupling elements, etc. Artificial-reality systems may also be configured with any other suitable type or form of image projection system, such as retinal projectors used in virtual retina displays. 
     The artificial-reality systems described herein may also include various types of computer vision components and subsystems. For example, the augmented-reality system  3100  and/or the virtual-reality system  3200  may include one or more optical sensors, such as two-dimensional (2D) or 3D cameras, structured light transmitters and detectors, time-of-flight depth sensors, single-beam or sweeping laser rangefinders, 3D LiDAR sensors, and/or any other suitable type or form of optical sensor. An artificial-reality system may process data from one or more of these sensors to identify a location of a user, to map the real world, to provide a user with context about real-world surroundings, and/or to perform a variety of other functions. 
     The artificial-reality systems described herein may also include one or more input and/or output audio transducers. Output audio transducers may include voice coil speakers, ribbon speakers, electrostatic speakers, piezoelectric speakers, bone conduction transducers, cartilage conduction transducers, tragus-vibration transducers, and/or any other suitable type or form of audio transducer. Similarly, input audio transducers may include condenser microphones, dynamic microphones, ribbon microphones, and/or any other type or form of input transducer. In some embodiments, a single transducer may be used for both audio input and audio output. 
     In some embodiments, the artificial-reality systems described herein may also include tactile (i.e., haptic) feedback systems, which may be incorporated into headwear, gloves, body suits, handheld controllers, environmental devices (e.g., chairs, floormats, etc.), and/or any other type of device or system. Haptic feedback systems may provide various types of cutaneous feedback, including vibration, force, traction, texture, and/or temperature. Haptic feedback systems may also provide various types of kinesthetic feedback, such as motion and compliance. Haptic feedback may be implemented using motors, piezoelectric actuators, fluidic systems, and/or a variety of other types of feedback mechanisms. Haptic feedback systems may be implemented independent of other artificial-reality devices, within other artificial-reality devices, and/or in conjunction with other artificial-reality devices. 
     By providing haptic sensations, audible content, and/or visual content, artificial-reality systems may create an entire virtual experience or enhance a user&#39;s real-world experience in a variety of contexts and environments. For instance, artificial-reality systems may assist or extend a user&#39;s perception, memory, or cognition within a particular environment. Some systems may enhance a user&#39;s interactions with other people in the real world or may enable more immersive interactions with other people in a virtual world. Artificial-reality systems may also be used for educational purposes (e.g., for teaching or training in schools, hospitals, government organizations, military organizations, business enterprises, etc.), entertainment purposes (e.g., for playing video games, listening to music, watching video content, etc.), and/or for accessibility purposes (e.g., as hearing aids, visual aids, etc.). The embodiments disclosed herein may enable or enhance a user&#39;s artificial-reality experience in one or more of these contexts and environments and/or in other contexts and environments. 
     By way of example and not limitation, the following embodiments are included in the present disclosure: 
     Example 1: A system for varifocal adjustments, which may include: a frame; an optical lens pair supported by the frame, including: a first lens that is movably coupled to the frame, the first lens having a first optical axis and being movable relative to the frame along the first optical axis; and a second lens that is fixedly coupled to the frame, the second lens having a second optical axis; and a flexure assembly configured to constrain movement of the first lens to a substantially linear pathway, wherein the flexure assembly includes at least one substantially planar flexure element having a first movable end portion coupled to the first lens and a second fixed end portion coupled to the frame. 
     Example 2: The system of Example 1, wherein the flexure assembly includes a first flexure element and a second flexure element that are separated from each other by a gap. 
     Example 3: The system of Example 2, wherein the gap is maintained by a first end block coupling a first end portion of the first flexure element to a first end portion of the second flexure element and by a second end block coupling a second end portion of the first flexure element to a second end portion of the second flexure element. 
     Example 4: The system of Example 2 or Example 3, wherein the flexure assembly further includes: a third flexure element positioned adjacent to the first flexure element, the first flexure element and third flexure element forming a first flexure pair; and a fourth flexure element positioned adjacent to the second flexure element, the second flexure element and fourth flexure element forming a second flexure pair. 
     Example 5: The system of any of Examples 1 through 4, wherein the at least one substantially planar flexure element includes a central portion having a greater thickness than end portions thereof. 
     Example 6: The system of Example 5, wherein the at least one substantially planar flexure element includes a base member and the central portion of the substantially planar flexure element includes a stiffener material coupled to the base member. 
     Example 7: The system of Example 6, wherein the stiffener material includes at least one of a polymer material or a metal material. 
     Example 8: The system of Example 6 or Example 7, wherein the base member has a thickness of about 0.003 inch or less. 
     Example 9: The system of any of Examples 6 through 8, wherein the stiffener material is coupled to only one side of the base member. 
     Example 10: The system of any of Examples 6 through 9, wherein the base member includes a metal material. 
     Example 11: The system of any of Examples 1 through 10, wherein the at least one flexure element has a shape defining a cutout to avoid optically obstructing light passing through the first lens parallel to the first optical axis. 
     Example 12: The system of any of Examples 1 through 11, further including an electronic display configured to display visual content to an intended user of the system for varifocal adjustments. 
     Example 13: The system of Example 12, wherein the first lens is positioned between the electronic display and the second lens. 
     Example 14: A head-mounted display system, which may include: an electronic display mounted to a frame; a first lens that is movably coupled to the frame, the first lens being movable relative to the frame; a second lens that is fixedly coupled to the frame, wherein the electronic display, first lens, and second lens are positioned for an intended user donning the head-mounted display system to view the electronic display through the first lens and the second lens; and a flexure assembly including at least one substantially planar flexure element having a first movable end portion coupled to the first lens and a second fixed end portion coupled to the frame. 
     Example 15: The system of Example 14, wherein the first lens is positioned between the electronic display and the second lens. 
     Example 16: The system of Example 14 or Example 15, wherein the at least one substantially planar flexure element of the flexure assembly includes a first flexure element set of multiple flexure elements and a second flexure element set of multiple flexure elements, wherein the first flexure element set and the second flexure element set are separated from each other by a gap. 
     Example 17: The system of any of Examples 14 through 16, wherein the at least one substantially planar flexure element of the flexure assembly includes a first flexure element positioned adjacent to a first lateral side of the first lens and a second flexure element positioned adjacent to a second, different lateral side of the first lens. 
     Example 18: The system of any of Examples 14 through 17, wherein the frame is a frame of a virtual-reality head-mounted display system. 
     Example 19: A method of forming a system for varifocal adjustments, which may include: fixedly coupling an electronic display and a stationary optical lens to a frame; positioning a movable optical lens between the electronic display and the stationary optical lens; and coupling a first movable end portion of a flexure assembly to the movable optical lens and coupling a second fixed end portion of the flexure assembly to the frame. 
     Example 20: The method of Example 19, wherein coupling the first movable end portion of the flexure assembly to the movable optical lens includes coupling a first end block of the flexure assembly to the movable optical lens and wherein coupling the second fixed end portion of the flexure assembly to the frame includes coupling a second end block of the flexure assembly to the frame. 
     Example 21: A system for varifocal adjustments, which may include: a frame; an optical lens pair supported by the frame, including: a first lens that is movably coupled to the frame, the first lens having a first optical axis and being movable relative to the frame along the first optical axis; and a second lens that is fixedly coupled to the frame, the second lens having a second optical axis; and a brake mechanism coupled to the frame and configured to frictionally stop movement of and maintain a position of the first lens along the first optical axis. 
     Example 22: The system of Example 21, wherein the brake mechanism includes at least one clamp arm positioned to apply a braking pressure against a mounting bracket of the first lens. 
     Example 23: The system of Example 22, wherein: the at least one clamp arm includes a first clamp arm and a second clamp arm; the mounting bracket includes a fin extending away from the mounting bracket; and the first clamp arm and the second clamp arm are positioned to apply a compressive force against the fin. 
     Example 24: The system of Example 22 or Example 23, wherein the brake mechanism further includes a brake actuator for moving the at least one clamp arm. 
     Example 25: The system of Example 24, wherein the brake actuator includes a wire coupled to the at least one clamp arm, wherein the wire is configured to apply a force to the at least one clamp arm to rotate the at least one clamp arm. 
     Example 26: The system of Example 25, wherein the wire includes a shape memory alloy and the wire has a length that is alterable upon application of a sufficient electrical voltage to the wire. 
     Example 27: The system of Example 25 or Example 26, wherein: the at least one clamp arm includes a first clamp arm and a second clamp arm; and the wire is coupled to a portion of each of the first clamp arm and the second clamp arm. 
     Example 28: The system of any of Examples 22 through 27, further including at least one spring positioned to bias the at least one clamp arm to a braking position. 
     Example 29: The system of any of Examples 22 through 28, further including at least one brake insert coupled to the at least one clamp arm in a position to apply the braking pressure against the mounting bracket. 
     Example 30: The system of Example 29, wherein the brake insert includes a polymeric material. 
     Example 31: The system of any of Examples 21 through 30, further including a voice coil actuator mounted to the frame and configured to move the first lens along the first optical axis. 
     Example 32: The system of any of Examples 21 through 31, further including a flexure assembly configured to guide movement of the first lens along the first optical axis. 
     Example 33: The system of any of Examples 21 through 32, wherein the brake mechanism includes a first clamp arm and a second clamp arm having a common axis of rotation that is substantially aligned with the first optical axis. 
     Example 34: A head-mounted display system, which may include: an electronic display element mounted to a frame; a first lens that is mounted on a mounting bracket and that is movably coupled to the frame, the first lens being movable relative to the frame; a second lens that is fixedly coupled to the frame, wherein the electronic display, first lens, and second lens are positioned for an intended user donning the head-mounted display system to view the electronic display through the first lens and the second lens; and a brake mechanism coupled to the frame and positioned to apply a braking pressure against the mounting bracket. 
     Example 35: The system of Example 34, further including a lens movement actuator including a base coupled to the frame and an output shaft coupled to the mounting bracket. 
     Example 36: The system of Example 35, wherein the brake mechanism includes a first clamp arm and a second clamp arm each rotatably coupled to the frame and configured to engage a respective surface of the mounting bracket to apply a braking pressure against the mounting bracket. 
     Example 37: The system of Example 36, wherein the brake mechanism includes a brake actuator configured such that actuation of the brake actuator results in releasing a brake pressure applied by the first clamp arm and the second clamp arm against the mounting bracket. 
     Example 38: The system of any of Examples 34 through 37, wherein the frame is a frame of a virtual-reality head-mounted display system. 
     Example 39: A method of making varifocal adjustments, which may include: moving a first lens relative to a frame supporting the first lens and relative to a second lens fixedly coupled to the frame, wherein the first lens is moved from a first position to a second position; maintaining the first lens in the second position by applying, with a brake mechanism, a braking pressure against a mounting bracket coupled to the first lens. 
     Example 40: The method of Example 39, wherein moving the first lens relative to the frame and relative to the second lens includes actuating a voice coil actuator. 
     The process parameters and sequence of the steps described and/or illustrated herein are given by way of example only and can be varied as desired. For example, while the steps illustrated and/or described herein may be shown or discussed in a particular order, these steps do not necessarily need to be performed in the order illustrated or discussed. The various example methods described and/or illustrated herein may also omit one or more of the steps described or illustrated herein or include additional steps in addition to those disclosed. 
     The preceding description has been provided to enable others skilled in the art to best utilize various aspects of the example embodiments disclosed herein. This example description is not intended to be exhaustive or to be limited to any precise form disclosed. Many modifications and variations are possible without departing from the spirit and scope of the present disclosure. The embodiments disclosed herein should be considered in all respects illustrative and not restrictive. Reference should be made to any claims appended hereto and their equivalents in determining the scope of the present disclosure. 
     Unless otherwise noted, the terms “connected to” and “coupled to” (and their derivatives), as used in the specification and/or claims, are to be construed as permitting both direct and indirect (i.e., via other elements or components) connection. In addition, the terms “a” or “an,” as used in the specification and/or claims, are to be construed as meaning “at least one of.” Finally, for ease of use, the terms “including” and “having” (and their derivatives), as used in the specification and/or claims, are interchangeable with and have the same meaning as the word “comprising.”