Abstract:
A lens assembly includes at least a pair of lenses fixed in a lens barrel and an additional lens initially having at least one degree of freedom of movement with respect to the other lenses. The additional lens can be fixed in a desired alignment position with respect to the other lenses.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/977,906, filed Apr. 10, 2014, the entirety of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to lens assemblies in general, and in particular, to miniature lens assemblies for use in miniature cameras and projectors. 
       BACKGROUND 
       [0003]    Various portable electronic devices such as cell phones and laptops use digital cameras to capture photographs and video and are expected to use digital projectors to display photographs or video on a screen or on an object. These are miniature digital cameras and include a miniature lens assembly, i.e., a lens assembly having a lens diameter of about 0.25 inches or less, which captures the light and focuses it onto a CMOS imager to capture an image. Inside a miniature projector, a lens assembly focuses the light from an LED array onto a screen or an object. The continuing demand for smaller and higher quality low cost imaging lens assemblies presents a considerable challenge to optical and mechanical design. The tiny lenses in the assembly should be aligned with respect to each other or a lens barrel within a few microns to ensure good image quality. Alignment errors between the lenses lead to a reduction in image quality. When the image quality of a lens assembly is not acceptable, the lens assembly is rejected. This leads to undesirable yield loss in the manufacturing of lens assemblies. 
         [0004]    To reduce lens alignment errors and improve manufacturing yields of lens assemblies, a variety of passive alignment methods have been devised. 
         [0005]    For example, referring to  FIGS. 1A and 1B , reproduced from FIGS. 2 and 4, respectively, of U.S. Pat. No. 7,088,530 entitled “Passively Aligned Optical Elements” to Recco et al. In  FIG. 1A , the alignment of two lenses L 1  and L 2  uses mating tapered surfaces  24  and  34 . In  FIG. 1B , lenses L 1  and L 3  are aligned to each other using the lens barrel  22 . Lenses in this lens assembly are tightly stacked inside the lens barrel  22  into predefined positions and are not allowed to move. 
         [0006]    As the resolution of a miniature camera increases, and the performance requirements for the lens assemblies become more stringent, the number of lenses in the lens assembly often increases. The increase in the number of optical elements that are stacked up tends to increase the impact of any alignment errors. As a result, the yield loss in the manufacture of lens assemblies using prior art passive alignment becomes worse when the number of lenses in the assembly increases. 
         [0007]    As lens assemblies become smaller, the amount of light collected by the lens assembly is reduced and lower f-number designs are required. The larger aperture designs magnify the sensitivity to lens alignment errors and the yield loss in the manufacture of lens assemblies using prior art passive alignment becomes worse. 
         [0008]    Active alignment of lenses is typically used for high performance optical systems where the cost of the active alignment is not an issue. However, known active alignment techniques, such as using an autocollimator and a rotational stage to individually align lenses can be too complex and costly for high-volume production of miniature lens assemblies. 
         [0009]    There is a need in the art for a low cost method of manufacturing lens assemblies for use in miniature cameras and miniature projectors that combines the performance advantages of active alignment and the low cost advantages of passive alignment. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0010]    These and other objects and advantages are achieved in accordance with the present invention by the lens assembly and method of making same described herein. 
         [0011]    According to an embodiment of the invention, a lens assembly includes multiple lenses; a lens barrel configured to receive and fixedly align the lenses; and at least one additional lens having at least one degree of freedom of movement with respect to the other lenses; wherein the additional lens(es) can be fixed in a desired alignment position with respect to the other lenses. 
         [0012]    According to another embodiment of the invention, a lens assembly includes a first plurality of lenses and a lens barrel configured to receive and fixedly align the first plurality of lenses. At least one additional lens having at least one degree of freedom of movement with respect to the first plurality of lenses is provided wherein the at least one additional lens is fixable in a desired alignment position with respect to the first plurality of lenses. 
         [0013]    According to a further embodiment of the invention, a method of making a lens assembly includes the steps of providing a first plurality of lenses, providing a lens barrel, receiving the first plurality of lenses in the lens barrel, and fixedly aligning the first plurality of lenses in the lens barrel. The method also includes providing at least one additional lens having at least one degree of freedom of movement with respect to the first plurality of lenses and fixing the at least one additional lens in a desired alignment position with respect to the first plurality of lenses. 
         [0014]    According to another embodiment of the invention, a lens assembly includes a plurality of lenses aligned inside of a lens barrel, wherein a movable lens is aligned to compensate for the errors in passive alignment of or inherent flaws in the other lenses in the lens assembly. In order to permit such active alignment, a “rattle” space is initially left around the movable lens so that it is not completely mechanically constrained and can be moved in at least one alignment degree of freedom. Other lenses in the lens assembly are mechanically constrained in all degrees of freedom using passive alignment such as, but not limited to, passive component alignment of lenses to each other or passive alignment of lenses to the lens barrel. 
         [0015]    In one embodiment, the movable lens has two alignment degrees of freedom orthogonal to the optical axis. 
         [0016]    In another embodiment, the movable lens has one alignment degree of freedom along the optical axis. In still another embodiment, the allowable movement of the movable lens is sufficiently small so that the optical performance of the lens assembly is good enough to ascertain lens quality before the movable lens is aligned. 
         [0017]    In a further embodiment, epoxy or additional adhesive is used to secure the movable lens after the active alignment is completed. In a still further embodiment, an alignment mechanism is used to passively align the movable lens to the lens barrel or to the other lenses, and the alignment mechanism can be removed at least partially when doing active alignment of the movable lens. 
         [0018]    In an alternative embodiment, the movable lens has a smooth interface with respect to the adjacent lens so that the movable lens can slide on the surface of the adjacent lens without a substantial change in tip, tilt or spacing between the lenses. 
         [0019]    In another embodiment, the movable lens has a smooth surface on its outer diameter so that the movable lens can slide along the lens barrel optical axis without a substantial change in tip, tilt, or decenter. In yet another embodiment, the movable lens is inside of a first portion of the lens barrel and the remaining lenses are inside of a second portion of the lens barrel and the two portions of the lens barrel are aligned and then fixed to each other in the aligned position. 
         [0020]    Another embodiment of the method for making a lens assembly comprises the steps of loading a plurality of lenses in a lens barrel, leaving some rattle space for a movable lens, adjusting the position of the movable lens to optimize the optical performance of the lens assembly, and securing the position of the movable lens. In a further embodiment, the method includes passively aligning the movable lens in order to measure the quality of the lenses prior to active alignment. 
         [0021]    The invention will be better understood with reference to the illustrative drawings and the detailed description of exemplary embodiments set forth as follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIGS. 1A and 1B  correspond to FIGS. 2 and 4 of prior art U.S. Pat. No. 7,088,530. 
           [0023]      FIG. 2  is a cross-sectional view of a lens assembly in accordance with one embodiment of the present invention. 
           [0024]      FIG. 3  is a cross-sectional view of a lens assembly in accordance with an additional embodiment of the present invention. 
           [0025]      FIG. 4  is a cross-sectional view of a lens assembly in accordance with a further embodiment of the present invention. 
           [0026]      FIG. 5  is a cross-sectional view of a lens assembly in accordance with a still further embodiment of the present invention. 
           [0027]      FIG. 6  is a cross-sectional view of a lens assembly in accordance with another embodiment of the present invention. 
           [0028]      FIG. 7  is a cross-sectional view of a lens assembly in accordance with yet another embodiment of the present invention. 
           [0029]      FIG. 8  is a cross-sectional view of a lens assembly in accordance with an additional embodiment of the present invention. 
           [0030]      FIG. 9  is a cross-sectional view of a lens assembly in accordance with a further embodiment of the present invention. 
           [0031]      FIG. 10  is a front view of the lens assembly of  FIG. 9 . 
           [0032]      FIG. 11  is a flowchart for a method of making a lens assembly in accordance with one embodiment of the present invention. 
           [0033]      FIG. 12  is a flowchart for a method of making a lens assembly in accordance with an additional embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    In accordance with embodiments further described herein, various lens assemblies are provided which may be used in miniature cameras or miniature projectors included in, for example, portable electronic devices such as cellphones. 
         [0035]    Referring now to the drawings, which are included for the purposes of illustrating embodiments of the invention, and not for limiting the same,  FIG. 2  shows a cross-sectional view of a lens assembly in accordance with one embodiment of the present invention. The lens assembly  120  is comprised of four lenses L 11 , L 12 , L 13  and L 14 , three baffles  123 ,  125 , and  127  and an IRCF (infra-red cut filter)  129  inserted in a lens barrel  121 . The baffles are interspersed between the lenses as shown. Lenses L 11 , L 12 , L 13  and L 14  are made of conventional lens material such as glass, plastic, optical crystal or the like. Baffles  123 ,  125 , and  127  are made of conventional baffle material such as plastic, cloth, paper, or the like. Lens barrel  121  is made of conventional lens barrel material such as plastic, metal, or the like. IRCF  129  is made of glass with an IR coating, plastic, or other conventional material. 
         [0036]    IRCF  129  filters out infra-red light while passing visible light in order to improve the color of the image captured by a CMOS image sensor (not shown). IRCF  129  is an optional part of the lens barrel assembly and both its presence and location depend on the camera design. Alternatively, IRCF  129  may be replaced with another conventional filter. 
         [0037]    The front of the lens barrel contains an aperture  122  on the front side, which serves as the entrance pupil for the imaging system. 
         [0038]    Lens L 12  is positioned inside lens barrel  121  in contact with an inner edge  103  and inside surface  104  of lens barrel  121 . Lens L 13  is adjacent to, and separated by baffle  125  from, lens L 12  and is also in contact with inside surface  104 . Lens L 14  is adjacent to, and separated by baffle  127  from, lens L 13  and is also in contact with inside surface  104 . IRCF  129  is positioned in contact with lens L 14  and inside surface  104 . The stack of optical elements is fixed inside the lens barrel by epoxy  109  that connects IRCF  129  with lens barrel  121 . 
         [0039]    As a result, lenses L 12 , L 13 , and L 14  are passively aligned in the lens assembly through physical contact among the lenses L 12 , L 13 , and L 14 , the baffles, and lens barrel  121 . Alternatively, lenses L 12 , L 13 , and L 14  could be aligned merely by connections between them. Depending on the method used and the dimensional tolerance of the lenses and lens barrel, the resulting optical alignment precision between lenses may be between less than 1 micron to over 15 microns in the x, y and z directions. 
         [0040]    Movable lens L 11  is positioned generally between aperture  122  and lens L 12 . Baffle  123  separates lenses L 11  and L 12 . Movable lens L 11  is not precisely positioned by the lens barrel  121  and there is a gap  101  that allows lens L 11  to be moved in the z direction and a radial gap  102  that allows it to be moved in the x and y directions. The gap  102  allows between 5 microns and 50 microns of movement by lens L 11  in the x and y directions, and preferably between 5 and 25 microns. Gap  101  may optionally be omitted. 
         [0041]    To reduce certain optical aberrations such as astigmatism, the movable lens L 11  may be aligned in the x and y directions so that its optical axis substantially coincides with the optical axis O of the lens assembly  120 . This may be done by monitoring the through focus MTF (modulation transfer function) of the lens assembly while the position of the movable lens L 11  is adjusted. The MTF measurement is known in the art and is typically performed by shining light through a mask that is placed at the image plane of the lens and monitoring the sharpness of the projected image with cameras. Through focus MTF measurement is also known in the art and consists of making multiple MTF measurements while changing the spacing between the lens assembly and the mask. Other conventional optical measurements of the lens assembly may be used for guiding the adjustment of the position of movable lens L 11 , including but not limited to, point spread function, line spread function, sharpness, contrast, brightness, spatial frequency response (SFR), subjective quality factor (SQF), and wave front measurements. 
         [0042]    Misalignment of lenses L 12 , L 13  and L 14  and imperfections in the lenses themselves will typically cause the optical axis O of the lens assembly to not coincide exactly with the optical axis of lens L 12 . An idealized optical axis O of the lens assembly is shown in  FIG. 2 . Adjusting the alignment of lens L 11  may be used to compensate, in whole or part, for such misalignment and imperfections or, alternatively, create desired optical effects. 
         [0043]    Alternatively, since movement of lens L 11  in the x and y directions also affects image plane tilt, the position of lens L 11  may be adjusted so as to align the optical axis O with the perpendicular of the imager (not shown) so that the entire image is in better focus. This may be done during manufacturing of the lens assembly or after the lens assembly is mounted in the camera. 
         [0044]    Once the movable lens L 11  is aligned in the desired position, epoxy  111  is used to fix it in position. As shown in  FIG. 2 , in a preferred embodiment, lens L 11  has a flat surface L 11 A and lens L 12  has a flat surface L 12 A and both surfaces L 11 A and L 12 A are in contact with baffle  123 . In an alternate embodiment, baffles  123 ,  125  and  127  are omitted allowing the lenses to contact each other directly at one or more interfaces. The baffles and interfaces between lenses are preferably designed to avoid allowing stray light to reach the imager (not shown). 
         [0045]      FIG. 3  shows a cross-sectional view of a lens assembly  130  in accordance with another embodiment of the present invention. The lens assembly  130  is comprised of four lenses L 21 , L 22 , L 23  and L 24 , and four baffles  132 ,  133 ,  135 , and  137  inserted in a lens barrel  131 . The baffles are interspersed between the lenses as shown. Lenses L 21 , L 22 , L 23  and L 24  are made of conventional lens material such as glass, plastic, optical crystal or the like. Baffles  133 ,  135 , and  137  are made of conventional baffle material such as plastic, cloth, paper, or the like. Lens barrel  131  is made of conventional lens barrel material such as plastic, metal, or the like. 
         [0046]    Lens L 24  is positioned inside lens barrel  131  in contact with a back edge and inside surface  304  of lens barrel  131 . Lens L 33  is adjacent to, and separated by baffle  137  from, lens L 24  and is also in contact with inside surface  304 . Lens L 22  is adjacent to, and separated by baffle  135  from, lens L 23  and is also in contact with inside surface  304 . 
         [0047]    As a result, lenses L 22 , L 23 , and L 24  are passively aligned in the lens assembly through physical contact among the lenses L 22 , L 23 , and L 24 , the baffles, and lens barrel  131 . Alternatively, lenses L 22 , L 23 , and L 24  could be aligned merely by connections between them. Depending on the method used and the dimensional tolerance of the lenses and lens barrel, the resulting optical alignment precision between lenses may be between less than 1 micron to over 15 microns in the x, y and z directions. 
         [0048]    Movable lens L 21  is positioned in front of lens L 22 . Baffle  133  separates lenses L 21  and L 22 . Movable lens L 21  is not precisely positioned by lens barrel  131 . Baffle  133  is positioned in contact with inside surface  304  of lens barrel  131  or by mating with a feature on the front surface of lens L 22  or on the back surface of lens L 21 . Front baffle  132  on the front side of lens L 21  serves as the entrance pupil for the imaging system and may be attached to lens L 21  using, for example, adhesive. There is a space  301  that allows lens L 21  to be moved in the z direction and a radial gap  302  that allows it to be moved in the x and y directions. Gap  302  allows movement of lens L 21  in the x and y directions by between 5 microns and 50 microns, and preferably, between 5 and 25 microns. 
         [0049]    To reduce certain optical aberrations such as astigmatism, the lens L 21  may be aligned in the x and y directions so that its optical axis substantially coincides with the optical axis O of the lens assembly  130 . This may be done by monitoring the through focus MTF (modulation transfer function) of the lens assembly while the position of lens L 21  is adjusted. Misalignment of lenses L 22 , L 23  and L 24  and imperfections in the lenses themselves will typically cause the optical axis O of the lens assembly to not coincide exactly with the optical axis of lens L 22 . An idealized optical axis O of the lens assembly is shown in  FIG. 3 . Adjusting the alignment of lens L 21  may be used to compensate, in whole or part, for such misalignment and imperfections or, alternatively, create desired optical effects. 
         [0050]    Alternatively, since movement of lens L 21  in the x and y directions also affects image plane tilt, the position of lens L 21  may be adjusted so as to align the optical axis O with the perpendicular of the imager (not shown) so that the entire image is in better focus. This may be done during manufacturing of the lens assembly or after the lens assembly is mounted in the camera. 
         [0051]    Once lens L 21  is aligned to the desired position, epoxy  311  is used to fix it in position with respect to lens L 22 . In an alternate embodiment, epoxy  311  may come in contact with lens barrel  131  and fix all lenses inside the lens barrel. As shown in  FIG. 3 , in a preferred embodiment, lens L 21  has a flat surface L 21 A and lens L 22  has a flat surface L 22 A and both surfaces L 21 A and L 22 A are in contact with baffle  133 . In an alternate embodiment, baffles  133 ,  135  and  137  are omitted allowing the lenses to contact each other directly at one or more interfaces. The baffles and interfaces between lenses are preferably designed to avoid allowing stray light to reach the imager (not shown). 
         [0052]      FIG. 4  shows a cross-sectional view of the lens assembly  130  of  FIG. 3  with a passive alignment ring  321  added to fill a portion of gap  302 . Passive alignment ring is preferably made of rubber, plastic, epoxy, metal, or other conventional material. Passive alignment ring  321  can be used to passively align movable lens L 21  into a position where the optical performance of the lens assembly is of sufficient quality to permit MTF measurements for determining whether more precise alignment of movable lens L 21  is warranted. Passive alignment ring  321  is preferably removed after initial MTF measurements are made and before active alignment of movable lens L 21 . Ring  321  is preferably omitted from a final lens assembly, as shown in  FIG. 3 . 
         [0053]    Alternatively, if the MTF measurements show that the lens assembly meets final requirements, passive alignment ring  321  may be left in place and remain present in the final lens assembly. As a result, some lens assemblies may have a passive alignment ring as illustrated in  FIG. 4  and some may not as illustrated in  FIG. 3 . If the passive alignment ring  321  is left on the lens assembly  130 , it may be preferably fixed to the lens barrel  131  using epoxy, welding, or another conventional method of attachment. 
         [0054]      FIG. 5  shows a cross-sectional view of a lens assembly  150  in accordance with still another embodiment of the present invention. The lens assembly  150  is comprised of four lenses L 31 , L 32 , L 33  and L 34 , an IRCF window  159 , and four baffles  152 ,  153 ,  155 , and  157  inserted in a lens barrel  151 . The baffles are interspersed between the lenses as shown. Lenses L 31 , L 32 , L 33  and L 34  are made of conventional lens material such as glass, plastic, optical crystal or the like. Baffles  153 ,  155 , and  157  are made of conventional baffle material such as plastic, cloth, paper, or the like. Lens barrel  151  is made of conventional lens barrel material such as plastic, metal, or the like. IRCF window  159  is made of glass with an IR coating, plastic, or other conventional material. 
         [0055]    IRCF window  159  filters out infra-red light while passing visible light in order to improve the color of the image captured by a CMOS image sensor (not shown). IRCF window  159  is an optional part of the lens barrel assembly and both its presence and location depend on the camera design. Alternatively, IRCF window  159  may be replaced with another conventional filter. 
         [0056]    IRCF window  159  is positioned inside lens barrel  131  in contact with a back edge and inside surface  504  of lens barrel  131 . Lens L 34  is in contact with IRCF window  159  and inside surface  504 . Lens L 33  is adjacent to, and separated by baffle  157  from, lens L 34  and is also in contact with inside surface  504 . Lens L 32  is adjacent to, and separated by baffle  155  from, lens L 33  and is also in contact with inside surface  504 . 
         [0057]    As a result, lenses L 32 , L 33 , and L 34  are passively aligned in the lens assembly through physical contact among the lenses L 32 , L 33 , and L 34 , the baffles, and lens barrel  151 . Alternatively, lenses L 32 , L 33 , and L 34  could be aligned merely by connections between them. Depending on the method used and the dimensional tolerance of the lenses and lens barrel, the resulting optical alignment precision between lenses may be between less than 1 micron to over 15 microns in the x, y and z directions. 
         [0058]    Movable lens L 31  is positioned in front of lens L 32 . Baffle  153  separates lenses L 31  and L 32 . Movable lens L 31  is not positioned within lens barrel  151 . Baffle  153  is adhered to, or mates with a feature on, the front surface of lens L 32  or the back surface of lens L 31 . Front baffle  152  on the front side of lens L 31  serves as the entrance pupil for the imaging system and may be attached to lens L 31  using, for example, adhesive. The movable lens L 31  lies substantially outside the lens barrel  151  and is free to move in the x, y and z directions. 
         [0059]    To reduce certain optical aberrations such as astigmatism, lens L 31  may be aligned in the x and y directions so that its optical axis substantially coincides with the optical axis O of the lens assembly  150 . This may be done by monitoring the through focus MTF (modulation transfer function) of the lens assembly while the position of lens L 31  is adjusted. Misalignment of lenses L 32 , L 33  and L 34  and imperfections in the lenses themselves will typically cause the optical axis O of the lens assembly to not coincide exactly with the optical axis of lens L 32 . An idealized optical axis O of the lens assembly is shown in  FIG. 5 . Adjusting the alignment of lens L 21  may be used to compensate, in whole or part, for such misalignment and imperfections or, alternatively, create desired optical effects. 
         [0060]    Alternatively, since movement of lens L 31  in the x and y directions also affects image plane tilt, the position of lens L 31  may be adjusted so as to align the optical axis O with the perpendicular of the imager (not shown) so that the entire image is in better focus. This may be done during manufacturing of the lens assembly or after the lens assembly is mounted in the camera. 
         [0061]    Once lens L 31  is aligned to the desired position, epoxy  511  is used to fix it in position with respect to lens L 32  and lens barrel  151  and fix all lenses inside the lens barrel. As shown in  FIG. 5 , in a preferred embodiment, lens L 31  has a flat surface L 31 A and lens L 32  has a flat surface L 32 A and both surfaces L 31 A and L 32 A are in contact with baffle  153 . In an alternate embodiment, baffles  153 ,  155  and  157  are omitted allowing the lenses to contact each other directly at one or more interfaces. The baffles and interfaces between lenses are preferably designed to avoid allowing stray light to reach the imager (not shown). 
         [0062]      FIG. 6  shows a cross-sectional view of the lens assembly  150  in  FIG. 5  with a lens cover  162  replacing the baffle  152 . Cover  162  preferably prevents stray light from entering the optical system through the sides of lens L 31 . Like baffle  152 , lens cover  162  may define an entrance pupil for the imaging system. Cover  162  is preferably made of injection molded plastic and attached to lens L 31  by interference fit, with adhesive, or like attachment. 
         [0063]      FIG. 7  shows a cross-sectional view of a lens assembly  170  in accordance with a further embodiment of the present invention. The lens assembly  170  is comprised of four lenses L 41 , L 42 , L 43  and L 44  and four baffles  172 ,  173 ,  175 , and  177 . The positions of the first movable lens L 41  and the second movable lens L 44  are adjustable in order to optimize the optical performance of the lens assembly  170 . The second lens L 42  and the third lens L 43  are passively aligned. The baffles are interspersed between the lenses as shown. Lenses L 41 , L 42 , L 43  and L 44  are made of conventional lens material such as glass, plastic, optical crystal or the like. Baffles  173 ,  175 , and  177  are made of conventional baffle material such as plastic, cloth, paper, or the like. Lens barrel  171  is made of conventional lens barrel material such as plastic, metal, or the like. 
         [0064]    Baffle  177  is positioned inside lens barrel  171  in contact with edge  703  and inside surface  704  of lens barrel  171 . Lens L 43  is positioned inside lens barrel  171  in contact with baffle  177  and inside surface  704  of lens barrel  171 . Lens L 42  is adjacent to, and separated by baffle  175  from, lens L 43  and is also in contact with inside surface  704 . Optionally, epoxy  712  attaches lens L 42  to inside surface  704  and fix the position of lenses L 42  and L 43 . 
         [0065]    As a result, lenses L 42  and L 43  are passively aligned in the lens assembly through physical contact among the lenses L 42  and L 43 , the baffles, and lens barrel  171 . Alternatively, lenses L 42  and L 43  could be aligned merely by connections between them. Depending on the method used and the dimensional tolerance of the lenses and lens barrel, the resulting optical alignment precision between lenses L 42  and L 43  may be between less than 1 micron to over 15 microns in the x, y and z directions. 
         [0066]    Movable lens L 41  is positioned in front of lens L 42 . Baffle  173  separates lenses L 41  and L 42 . Movable lens L 41  is positioned in lens barrel  171  but is not precisely positioned. Radial gap  702  allows movement of lens L 41  in the x and y directions by between 5 microns and 50 microns, and preferably, between 5 and 25 microns. Baffle  173  is adhered to, mates with a feature on, or is aligned with a recessed feature on, the front surface of lens L 42  or the back surface of lens L 41 . 
         [0067]    The front baffle  172  is attached to the front surface of the movable lens L 41  using, for example, adhesive. Baffle  172  has an aperture  172 A that defines the entrance pupil for the imaging system. The entrance pupil may also be formed by baffle  173  or another aperture in the system. There is a space  701  that allows lens L 41  to be moved in the z direction. 
         [0068]    Movable lens L 44  is positioned in lens barrel  171  behind lens L 43  and baffle  177  but is not precisely positioned. Gap  705  separates baffle  177  and lens L 44  and allows movable lenses L 44  to be moved in the z direction adjusting its spacing with respect to fixed lens L 43 . The position of lens L 44  in the x and y directions and its tilt about the x and y axes is passively set by contact with the inside surface  704  of lens barrel  171 . 
         [0069]    In an alternate embodiment, there is a radial gap between lens L 44  and lens barrel  171  to allow the position and tilt of lens L 44  to be adjusted in the x and y directions. 
         [0070]    To reduce certain optical aberrations such as astigmatism, lens L 41  may be aligned in the x and y directions so that its optical axis substantially coincides with the optical axis O of the lens assembly  170 . This may be done by monitoring the through focus MTF (modulation transfer function) of the lens assembly while the position of lens L 41  is adjusted. Misalignment of lenses L 42 , L 43 , and/or L 44  and imperfections in the lenses themselves will typically cause the optical axis O of the lens assembly to not coincide exactly with the optical axis of lens L 42 . An idealized optical axis O of the lens assembly is shown in  FIG. 7 . Adjusting the alignment of lens L 41  may be used to compensate, in whole or part, for such misalignment and imperfections or, alternatively, create desired optical effects. 
         [0071]    Alternatively, since movement of lens L 41  in the x and y directions also affects image plane tilt, the position of lens L 41  may be adjusted so as to align the optical axis O with the perpendicular of the imager (not shown) so that the entire image is in better focus. This may be done during manufacturing of the lens assembly or after the lens assembly is mounted in the camera. 
         [0072]    Once lens L 41  is aligned to the desired position, epoxy  711  is used to fix it in position with respect to lens L 22 . In an alternate embodiment, epoxy  711  may come in contact with lens barrel  171  and fix lenses L 41 , L 42 , and L 43  inside the lens barrel. As shown in  FIG. 7 , in a preferred embodiment, lens L 41  has a flat surface L 41 A and lens L 42  has a flat surface L 42 A and both surfaces L 41 A and L 42 A are in contact with baffle  173 . In an alternate embodiment, baffles  173 ,  175  and  177  are omitted, allowing the lenses to contact each other directly at one or more interfaces. The baffles and interfaces between lenses are preferably designed to avoid allowing stray light to reach the imager (not shown). 
         [0073]    To reduce certain optical aberrations such as field curvature, movable lens L 44  is preferably aligned in the z direction so as to set optimum spacing between lenses L 43  and L 44 . Once movable lens L 44  is aligned in the desired position, epoxy  713  is used to fix it in position. 
         [0074]    Alternatively, additional lenses and baffles may be included in the lens assembly to achieve the desired optical performance, and less than four lenses and four baffles may be used to reduce cost. 
         [0075]    In this example, movement of lens L 41  in the x and y directions strongly affects image plane tilt and astigmatism, while movement of lens L 44  in the z direction strongly affects field curvature. To determine which lenses to actively align and in what direction, a sensitivity analysis can be done on the specific optical design to determine which lenses have large contribution on the aberration that needs to be corrected. More generally, specific optical aberrations can be induced or corrected by adjusting the positions of lenses L 41  and L 44  to obtain a desired optical performance of the lens assembly. 
         [0076]      FIG. 8  shows a cross-sectional view of a lens assembly  180  in accordance with yet another embodiment of the present invention. The lens assembly  180  is comprised of four lenses L 51 , L 52 , L 53  and L 54  and three baffles  183 ,  185 , and  187  inserted in a lens barrel  181 . The baffles are interspersed between the lenses as shown. Lenses L 51 , L 52 , L 53  and L 54  are made of conventional lens material such as glass, plastic, optical crystal or the like. Baffles  183 ,  185 , and  187  are made of conventional baffle material such as plastic, cloth, paper, or the like. Lens barrel  181  is made of conventional lens barrel material such as plastic, metal, or the like. 
         [0077]    Lens L 52  is positioned inside lens barrel  181  in contact with an inner edge  803  and inside surface  804  of lens barrel  121 . Lens L 53  is adjacent to, and separated by baffle  185  from, lens L 52  and is also in contact with inside surface  804 . Lens L 54  is adjacent to, and separated by baffle  187  from, lens L 53  and is also in contact with inside surface  804 . The stack of optical elements is fixed inside the lens barrel by epoxy  809  that connects lens L 54  with lens barrel  121 . Baffle  182  defines an aperture which serves as the entrance pupil for the imaging system. Baffle  182  is optionally attached to the front of lens L 51  using epoxy  810  and/or attached to lens barrel  181  using epoxy  811 . 
         [0078]    As a result, lenses L 52 , L 53 , and L 54  are passively aligned in the lens assembly through physical contact among the lenses L 52 , L 53 , and L 54 , the baffles, and lens barrel  181 . Alternatively, lenses L 52 , L 53 , and L 54  could be aligned merely by connections between them. Depending on the method used and the dimensional tolerance of the lenses and lens barrel, the resulting optical alignment precision between lenses may be between less than 1 micron to over 15 microns in the x, y and z directions. 
         [0079]    Movable lens L 51  is positioned in front of lens L 52 . Baffle  183  separates lenses L 51  and L 52 . Movable lens L 51  is not precisely positioned by lens barrel  181  and there is a gap  801  that allows movable lens L 51  to be moved in the z direction and a radial gap  802  that allows it to be moved in the x and y directions. The gap  802  allows between 5 microns and 50 microns of movement by lens L 51  in the x and y directions, and preferably between 5 and 25 microns. The position of lens L 51  is preferably fixed with respect to lens barrel  181  by the combination of baffle  182 , epoxy  810 , and epoxy  811 . Gaps  801  or  802  may optionally be omitted. 
         [0080]    To reduce certain optical aberrations such as astigmatism, the movable lens L 51  may be aligned in the x and y directions so that its optical axis substantially coincides with the optical axis O of the lens assembly  180 . This may be done by monitoring the through focus MTF (modulation transfer function) of the lens assembly while the position of the movable lens L 51  is adjusted. Other conventional optical measurements of the lens assembly may be used for guiding the adjustment of the position of movable lens L 51 , including but not limited to, point spread function, line spread function, sharpness, contrast, brightness, spatial frequency response (SFR), subjective quality factor (SQF), and wave front measurements. 
         [0081]    Misalignment of lenses L 52 , L 53  and L 54  and imperfections in the lenses themselves will typically cause the optical axis O of the lens assembly to not coincide exactly with the optical axis of lens L 52 . An idealized optical axis O of the lens assembly is shown in  FIG. 8 . Adjusting the alignment of lens L 51  may be used to compensate, in whole or part, for such misalignment and imperfections, or, alternatively, create desired optical effects. 
         [0082]    Alternatively, since movement of lens L 51  in the x and y directions also affects image plane tilt, the position of lens L 51  may be adjusted so as to align the optical axis O with the perpendicular of the imager (not shown) so that the entire image is in better focus. This may be done during manufacturing of the lens assembly or after the lens assembly is mounted in the camera. 
         [0083]    In a further alternate embodiment, once the movable lens L 51  is aligned in the desired position, epoxy (not shown) between lens L 51  and inner surface  804  is used to fix it in position. As shown in  FIG. 8 , in a preferred embodiment, lens L 51  has a flat surface L 51 A and lens L 52  has a flat surface L 52 A and both surfaces L 51 A and L 52 A are in contact with baffle  183 . In an alternate embodiment, baffles  183 ,  185  and  187  are omitted allowing the lenses to contact each other directly at one or more interfaces. The baffles and interfaces between lenses are preferably designed to avoid allowing stray light to reach the imager (not shown). 
         [0084]      FIG. 9  shows a cross-sectional view of a lens assembly in accordance with an embodiment of the present invention similar to that described in connection with  FIG. 2 . The lens assembly  190  is comprised of five lenses L 61 , L 62 , L 63 , L 64 , and L 65 ; three baffles  193 ,  195 , and  197 ; lens spacer  199 ; and IRCF  194  inserted in a lens barrel  191 . The baffles and the lens spacer are interspersed between the lenses as shown. Lenses L 61 , L 62 , L 63 , L 64  and L 65  are made of conventional lens material such as glass, plastic, optical crystal or the like. Baffles  193 ,  195 , and  197  are made of conventional baffle material such as plastic, cloth, paper, or the like. Lens barrel  191  is made of conventional lens barrel material such as plastic, metal, or the like. IRCF  194  is made of glass with an IR coating, plastic, or other conventional material. Lens spacer  199  is made of plastic, rubber, metal, or the like. 
         [0085]    Movable lens L 61  is not precisely positioned by lens barrel  191  and there is a gap  901  that allows lens L 61  to be moved in the z direction and a radial gap  902  that allows it to be moved in the x and y directions. The gap  902  allows between 5 microns and 50 microns of movement by lens L 61  in the x and y directions, and preferably between 5 and 25 microns. Gap  901  may optionally be omitted. 
         [0086]    The front surface of lens barrel  191  includes preferably three openings  192  which permit access to movable lens L 61  for performing active alignment of lens L 61  with the stack of lens L 62 , L 63 , L 64  and L 65  or for fixing the position of lens L 61  in a desired position. Lens L 61  is moved via the holes  192  into the desired alignment position and epoxy is inserted through holes  192  to fix lens L 61  in the desired position. 
         [0087]      FIG. 10  is a top view of lens assembly  190  of  FIG. 9 . For clarity,  FIG. 9  is a cross-sectional view of lens assembly  190  taken across Line AA of  FIG. 10 . 
         [0088]      FIG. 11  is a flowchart for a method of making a lens assembly in accordance with an embodiment of the present invention. In step  1001 , a lens barrel is provided and a plurality of lenses, including at least one movable lens, and other optical elements are inserted in the lens barrel. The lenses that are not movable are aligned with each other or in a fixed position relative to the lens barrel. 
         [0089]    In step  1002  at least one of the passively aligned lenses are fixed in position to prevent motion during latter steps. Step  1002  is optionally omitted, for example, if the passively aligned lenses are not able to move due to a tight fit with the lens barrel or are held in position by an additional component such as a retainer ring, IRCF window, lens, epoxy, or other optical or mechanical structure or the like. 
         [0090]    In step  1003  the movable lens is passively aligned temporarily. This may be done using a passive alignment ring as previously described in reference to  FIG. 4 , or with a fixture that comes down on the lens assembly to align the movable lens with respect to the rest of the lens assembly. Step  1003  is optionally omitted. 
         [0091]    In step  1004 , at least one optical characteristic of the lens assembly is measured. For example, a MTF measurement may be performed by shining light through a mask that is placed at the image plane of the lens assembly and monitoring the projected image with cameras placed at various field locations, e.g., at center and the four corners at 80% field. Alternatively, a through focus MTF measurement may be performed by making multiple MTF measurements at different field positions while changing the spacing between the lens assembly and the mask. Other measurements of the imaging quality of the lens assembly may be used, including but not limited to, point spread function, line spread function, sharpness, contrast, brightness, spatial frequency response (SFR), subjective quality factor (SQF), and wave front measurement. 
         [0092]    In step  1005 , the measured optical characteristic of the lens assembly is compared with an initial specification for pass/fail decision. If the part fails, it is rejected in step  1010 . If the part passes, it moves on to active alignment. This initial specification may not be as stringent as the final requirements for the lens assembly, but should determine that the optical elements in the lens assembly are of sufficient quality to warrant the effort of active alignment. For example, when using a through focus MTF measurement, different field positions may reach peak MTF at different positions of the mask with respect to the lens assembly, such as is the case of image plane tilt or field curvature. Furthermore, the tangential and sagittal MTF curves at a given field position may not be aligned, such as is the case for astigmatism. 
         [0093]    Using programs such as Zeemax or Code V, one skilled in the art can determine the effect that movement of the movable lens within the range allowable in the lens assembly can have on the through focus MTF curves. For example, in a lens assembly where movement of the first lens in the x and y directions (orthogonal to the optical axis) significantly affects astigmatism and image plane tilt, but does not significantly affect field curvature or the peak MTF for any through focus MTF curve, the initial specification may be to have a minimum requirement for peak MTF for each curve, regardless of misalignment, since adjustments of the movable lens in active alignment will not be able to substantially increase the peak MTF for each through focus curve. The initial specification may also include a minimum requirement for field curvature, since adjustments of the movable lens in active alignment will not be able to substantially reduce field curvature. 
         [0094]    In step  1007 , the active alignment of the movable lens is performed. Preferably, the movable lens is held with, for example a first gripper, and the rest of the lens assembly is held with, for example a second gripper. The first gripper position is modified with respect to the second gripper position to adjust the position of the movable lens in the lens assembly while the imaging quality is monitored. Once a desired or optimum position for the movable lens is found, the optical characteristic of the lens assembly is compared with a final specification. If the optical characteristic does not meet the final specification, the lens assembly is rejected in step  1010 . If the characteristic of the lens assembly meets the final specification, the movable lens is fixed in position in step  1009  using, for example but not limited to, an epoxy that hardens when exposed to UV light, pressure sensitive adhesive, laser welding, or localized melting. 
         [0095]    In an alternate embodiment, the method of  FIG. 11  may be simplified by eliminating steps  1002 ,  1003 ,  1004 , and  1005 , allowing step  1007  to proceed after step  1001 . Such omissions may be acceptable in the event that, for example, the performance of the lens assembly is known to be of sufficient quality by employing other quality control processes. 
         [0096]      FIG. 12  is a flowchart for a method of lens assembly in accordance with another embodiment of the present invention. Steps identified with the same numbers have the same function as described in connection with  FIG. 11 . 
         [0097]    In step  1006 , the optical characteristic of the lens assembly is compared with the final specification to determine if active alignment in step  1007  is even needed. If the optical characteristic, such as imaging quality, of the lens assembly measured in step  1004  meets the final specification in step  1006 , steps  1007  and  1008  are skipped and the movable lens position is fixed in step  1009 . Alternatively, if the position of the movable lens has already been temporarily fixed, for example with the passive alignment ring  321  previously described in reference to  FIG. 4 , the lens assembly may be secured in place using, for example but not limited to, epoxy, adhesive, laser welding, or localized melting. 
         [0098]    While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by way of example only, and not of limitation. Likewise, the various diagrams may depict an example architectural or other configuration for the disclosure, which is done to aid in understanding the features and functionality that can be included in the disclosure. The disclosure is not restricted to the illustrated example architectures or configurations, but the desired features can be implemented using a variety of alternative architectures and configurations. Indeed, it will be apparent to one of skill in the art how alternative functional, logical or physical partitioning and configurations can be implemented to implement the desired features of the present disclosure. Additionally, with regard to flow diagrams, operational descriptions and method claims, the order in which the steps are presented herein shall not mandate that various embodiments be implemented to perform the recited functionality in the same order unless the context dictates otherwise. 
         [0099]    Although the disclosure is described above in terms of various example embodiments and implementations, it should be understood that the various features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations, to one or more of the other embodiments of the disclosure, whether or not such embodiments are described and whether or not such features are presented as being a part of a described embodiment. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described example embodiments, and it will be understood by those skilled in the art that various changes and modifications to the previous descriptions may be made within the scope of the claims.