Abstract:
An endoscope is provided having a shaft and an optical system disposed in the shaft. The optical system defines an optical path. The optical system includes a first relay lens and a first meniscus lens positioned in the optical path and between an intermediate image plane and the first relay lens. In one embodiment, a second relay lens and a second meniscus lens, the first relay lens and the first meniscus lens residing on a first side of the intermediate image plane, and the second relay lens and the second meniscus lens residing on a second side of the intermediate image plane, wherein the first and second sides of the intermediate image plane are opposing sides.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to rod lens relay systems, which are often employed in medical endoscopes. 
       BACKGROUND OF THE INVENTION 
       [0002]    Relay lenses, such as rod lenses, are often used in endoscopes to relay the image from the objective. A relay lens can include a “relay pair” that includes two rod lenses, and multiple relay lenses can be used to extend the distance from the objective to the image. That is, the length of the shaft of an endoscope can be increased by using relay lenses. Typically, an odd number of relay lenses are used in an endoscope in order to generate a right-side-up image at the eyepiece of the endoscope. 
         [0003]      FIG. 1  shows a prior art relay lens  100  comprising rod lens doublets  101  and  102 . Rod lenses  101  and  102  reimage an image formed at image plane  103  to image plane  104  along the optical path from the distal end  107  to the proximal end  108  of the relay lens  100 . Image plane  103  resides between an objective lens (not shown) and rod lens  101 , while image plane  104  resides between rod lens  102  and an eye piece or focusing lens (not shown), either of which can present a final image to a sensor. Alternatively, image planes  103  and  104  can be intermediate image planes which reside between relay lens  100  and additional relay lenses or other optical elements. For example, image plane  103  can be an intermediate image plane containing an image formed by an objective lens or rod lens relay and image plane  104  can contain a further intermediate image or a final image. 
         [0004]    Conventional relay lenses contribute large amounts of astigmatism and field curvature to the image. These aberrations have traditionally been corrected in the objective optical elements. However, such correction of the aberrations solely in the objective elements leads to designs that are highly sensitive to tilt and decenter errors caused by manufacturing tolerances. For an endoscope with a large number of relays, the aberrations caused by the stack-up of these tilt and decenter errors in manufacturing tolerances can be too severe to correct solely in the objective elements. In addition, burdening the objective elements with excessive aberration correcting properties can increase the required surface curvatures and/or the optical work done by the objective elements. This can further increase tolerance sensitivity of the system. 
         [0005]    There are other ways to compensate for the aberrations in rod lens relays. These include using the eyepiece or a combination of the eyepiece and the relays. Another solution is to design relays with fewer aberrations. U.S. Pat. Nos. 4,676,606, 4,693,568, and 7,515,355 each include information regarding aberration compensation. Many lens system designs require that any aberration correction elements be designed in conjunction with the relay lenses, which restricts the usefulness of the aberration correction elements and increases the cost of designing each lens system. 
         [0006]    What is needed, therefore, is a way to effectively correct for aberrations in the relay lenses without over-burdening the objective elements and to reduce the tolerance sensitivity of the lens system. It is further desired to improve the compatibility of aberration correction systems with various rod lens systems. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the invention to more effectively correct for aberrations in a relay lens. 
         [0008]    It is a further object of the invention to provide correction for aberrations in rod lens relays that is compatible with rod lens relays of multiple types. 
         [0009]    It is a further object of the invention to provide correction for aberrations in rod lens relays in a cost-effective manner. 
         [0010]    The foregoing and other objects are at least partially achieved by provision of various embodiments of the present invention. Aberrations in one or more relay lenses are at least partially compensated for by introducing at least one afocal meniscus lens between an image plane and at least one of the relay lenses. Image plane refers to a plane containing an image formed by an element of an optical system. Example elements include, for example, an objective lens and relay lens, such as a rod lens relay (e.g., Hopkins rod-lens system). 
         [0011]    According to an aspect of the present invention, an endoscope is provided having a shaft and an optical system disposed in the shaft. The optical system defines an optical path. The optical system includes a first relay lens and a first meniscus lens positioned in the optical path and between an intermediate image plane and the first relay lens. 
         [0012]    According to another aspect of the present invention, an optical system for use in an endoscope is provided. The optical system defines an optical path and includes a first relay lens and a first meniscus lens positioned in the optical path and between an intermediate image plane and the first relay lens. 
         [0013]    In addition to, or as an alternative to, one or more of the features described above, further aspects of the present invention can include one or more of the following features, individually or in combination:
       the optical system includes a second relay lens and a second meniscus lens; the first relay lens and the first meniscus lens reside on a first side of the intermediate image plane; the second relay lens and the second meniscus lens reside on a second side of the intermediate image plane; the first and second sides of the intermediate image plane are opposing sides;   the first and second meniscus lenses are consecutively arranged along the optical path;   the first relay lens has a relay lens pair;   the second relay lens has a relay lens pair;   the relay lens pair has a first rod lens and a second rod lens;   the first meniscus lens is substantially afocal;   the first meniscus lens has a first surface facing the first relay lens and an opposing second surface facing the intermediate image plane, the first surface being convex;   the second meniscus lens has a first surface facing the second relay lens pair and an opposing second surface facing the intermediate image plane, the first surface being convex;   the first meniscus lens has a first surface and an opposing second surface; the first meniscus lens defines a radius, and a thickness extending between the first surface and the second surface; the ratio of the radii of the first surface of the first meniscus lens and the second surface of the first meniscus lens is about 0.75; the ratio of the radius of the first surface to the thickness of the first meniscus lens is about 1.35;   the first meniscus lens has a first surface and an opposing second surface; the first meniscus lens defines a radius, and a thickness extending between the first surface and the second surface; the second meniscus lens has a first surface and an opposing second surface; the second meniscus lens defines a radius, and a thickness extending between the first surface and the second surface; the ratio of the radii of the first surface of the first meniscus lens and the second surface of the first meniscus lens is about 0.75; the ratio of the radius of the first surface to the thickness of the first meniscus lens is about 1.35; the ratio of the radii of the first surface of the second meniscus lens and the second surface of the second meniscus lens is about 0.75; the ratio of the radius of the first surface to the thickness of the first meniscus lens is about 1.35;   an index of refraction of the first meniscus lens is between about 1.7 and about 1.9.   the first meniscus lens is a doublet having a first lens proximal to the intermediate image plane and a second lens distal from the intermediate image plane;   the first lens has a ratio of its radius to a total thickness of the doublet ratio of about 1.55:1 and the second lens has a radius to doublet thickness ratio of about 1.2:1;   the doublet comprises a first lens having an Abbe number of about 47 and a second lens having an Abbe number of about 24;   the optical system has a Hopkins rod lens system; the Hopkins rod lens system including the first relay lens;   the optical system has a Hopkins rod lens system, the Hopkins rod lens system including the first and second relay lenses; and   the first lens is a plano-convex lens and the second lens is a plano-concave lens.       
 
         [0031]    These and other aspects of the present invention will become apparent in light of the drawings and detailed description provided below. 
         [0032]    An advantage of the present invention is that the afocal meniscus lenses can be used with existing rod lens systems to improve the image quality. Menisci and relay lenses can be mixed and matched for optimum correction of aberrations in the relay lenses. Different designs of afocal meniscus lenses will produce different aberration compensation and correction results. 
         [0033]    Other objects, features, and advantages will be apparent from the following detailed description of embodiments of the present invention taken in conjunction with the accompanying drawings. For example, although rod lens relays are shown in the drawings, the present invention includes embodiments utilizing afocal meniscus lens to correct for aberrations in other types of optical relay systems (e.g., a conventional biconvex lens relay system). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  shows a schematic elevation view of a prior art optical system. 
           [0035]      FIG. 2  shows a schematic elevation view of a first embodiment of the invention. 
           [0036]      FIG. 3  shows a schematic elevation view of a second embodiment of the invention. 
           [0037]      FIG. 4  shows a schematic elevation view of a third embodiment of the invention. 
           [0038]      FIG. 5  shows a schematic elevation view of a fourth embodiment of the invention, with an endoscope  510  shown schematically in outline. 
           [0039]      FIG. 6  shows a schematic elevation view of a fifth embodiment of the invention. 
           [0040]      FIG. 7  shows a schematic elevation view of a sixth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    The invention will now be described in reference to  FIGS. 2-7 , which show exemplary embodiments of the present invention. 
         [0042]      FIG. 2  shows a first exemplary embodiment of an optical system  250  defining an optical path and including a relay lens  200 , an objective lens  220  at the distal end of the relay lens  200 , and an eyepiece  230  at the proximal end of the relay lens  200 . The relay lens  200  includes a first rod lens  201  and a second rod lens  202 . In the embodiment shown in  FIG. 2 , the rod lenses  201 ,  202  are doublets, which each consist of two simple lenses with a shared surfaces  201   c ,  202   c . In other embodiments, however, rod lenses that consist of only a single lens, or compound rod lenses comprising any number of simple lenses, are used. Between the rod lenses  201 ,  202  in  FIG. 2  is a stop  207 . The stop  207  is located in the afocal space of the relay lens  200 , in which the rays of the image transmitted through the optical system  250  undergo no substantial net convergence or divergence. In some embodiments, the stop can be an aperture stop. As used herein, “stop” refers to openings or structures that limit ray bundles (e.g., an aperture stop). An aperture stop is a stop that determines the ray cone angle, or equivalently the brightness, at an image point. 
         [0043]    In the embodiment of  FIG. 2 , each rod lens  201 ,  202  has an image plane  203 ,  204  associated with it. The respective image planes  203 ,  204  are on opposite sides of the rod lenses  201 ,  202 . A first meniscus lens  205  is positioned along the optical path between the image plane  203  and the first rod lens  201  and a second meniscus lens  206  is positioned along the optical path between image plane  204  and second rod lens  202 . 
         [0044]    In the embodiment of  FIG. 2 , both the rod lenses  201 ,  202  and the meniscus lenses  205 ,  206  have distal and proximal surfaces  201   a - b ,  202   a - b ,  205   a - b , and  206   a - b . Rod lenses  201 ,  202  also have shared surfaces  201   c ,  202   c  between the two simple lenses. Further, both the first and second meniscus lenses  205 ,  206  are substantially afocal. That is, the first and second meniscus lenses  205 ,  206  are designed so as to produce no substantial net convergence or divergence of collimated light. The meniscus lenses  205  and  206  serve to at least partially compensate for astigmatism and field curvature introduced into the optical system  250  by, for example, the rod lenses  201  and  202 . 
         [0045]    In some embodiments, including the embodiment shown in  FIG. 2 , the first meniscus lens  205  and the second meniscus lens  206  can be identical lenses oriented in opposite directions in the relay. Further, the first and second meniscus lenses  205 ,  206 , which are substantially afocal, can be configured to correct aberrations introduced into the system by the rod lenses. These include astigmatism and field curvature. 
         [0046]    The ratio of the radii of the curved surfaces for each of the first and second meniscus lenses  205 ,  206  can be about 0.75 or 3:4. In other embodiments, this ratio can be different and can be varied alongside the other characteristics of the lens, such as its thickness, index of refraction, Abbe number, etc. The characteristics of the meniscus lenses  205 ,  206  can be selected so that the meniscus lenses  205 ,  206  are afocal. It is notable that, because the meniscus lenses  205 ,  206  are afocal, it is possible to use the relay lens  200  to relay an image without the meniscus lenses  205 ,  206 . Likewise, the same meniscus lenses  205 ,  206  can be used with other rod lens relay designs to compensate for aberrations. The invention thus provides increased flexibility for designers—afocal meniscus lenses of different designs can be mixed and matched with rod lens relays of different designs. 
         [0047]    In an embodiment similar to that shown in  FIG. 2 , the elements of the optical system can have the following characteristics: 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Distance to 
                   
                   
                   
               
               
                   
                   
                 Next Surface 
                   
                   
                 Outer 
               
               
                   
                 Radius 
                 or Thickness 
                 Index of 
                 Abbe 
                 Diameter 
               
               
                 Surface 
                 (mm) 
                 (mm) 
                 Refraction 
                 Number 
                 (mm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Objective 
                 Infinity 
                 0.6185 
                 — 
                 — 
                 — 
               
               
                 First meniscus, distal 
                 −3.2446 
                 2.4046 
                 1.788 
                 47.3 
                 1.8 
               
               
                 First meniscus, proximal 
                 −4.3062 
                 0.1 
                 — 
                 — 
                 2.79 
               
               
                 First rod lens, proximal 
                 8.0175 
                 19.2 
                 1.603 
                 38.0 
                 2.79 
               
               
                 First rod lens, shared 
                 5.4772 
                 1.8 
                 1.547 
                 53.6 
                 2.79 
               
               
                 First rod lens, distal 
                 −8.0832 
                 0.077 
                 — 
                 — 
                 2.79 
               
               
                 STOP 
                 Infinity 
                 0.077 
                 — 
                 — 
                 2.79 
               
               
                 Second rod lens, distal 
                 8.0832 
                 1.8 
                 1.547 
                 53.6 
                 2.79 
               
               
                 Second rod lens, shared 
                 −5.4772 
                 19.2 
                 1.603 
                 38.0 
                 2.79 
               
               
                 Second rod lens, proximal 
                 −8.0175 
                 0.1 
                 — 
                 — 
                 2.79 
               
               
                 Second meniscus, distal 
                 4.3062 
                 2.4046 
                 1.788 
                 47.3 
                 2.79 
               
               
                 Second meniscus, proximal 
                 3.2446 
                 0.6185 
                 — 
                 — 
                 1.8 
               
               
                 Image 
                 Infinity 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
         [0048]    In the optical system having the characteristics shown in the foregoing chart, the objective is at the distal end of the relay lens, and each successive surface is the next surface in the proximal direction until the proximal-most image plane is reached. The distances and length measurements are in millimeters. 
         [0049]    In the optical system having the characteristics shown in the foregoing chart, the first meniscus lens&#39; distal surface is concave, has a radius of 3.2446 millimeters, an outer diameter of 1.8 millimeters, and is located 0.6185 millimeter from the objective. The thickness of the first meniscus lens is 2.4046 millimeters. The proximal-most surface of the first meniscus lens has a radius of 4.3062 millimeters and an outer diameter of 2.79 millimeters. The first meniscus lens has an index of refraction of 1.788 and an Abbe number of 47.3. The plano-bevel on the distal concave surface of the first meniscus lens is 1.8 millimeters. 
         [0050]    The first meniscus lens is 0.1 millimeter from the first rod lens of the relay lens, which has two elements. The distal-most surface of the distal-most element of the first rod lens has a radius of 8.0175 millimeters and is 19.2 millimeters thick. The surface shared by the two elements of the rod lens has a radius of 5.4772 millimeters and is separated from the distal-most surface of the first rod lens by 1.8 millimeters. The proximal most surface of the first rod lens has a radius of 8.0832 millimeters. The distal-most element of the first rod lens has an index of refraction of 1.603 and an Abbe number of 38.0, while the proximal-most element of the rod lens  201  has an index of refraction of 1.547 and an Abbe number of 53.6. 
         [0051]    In the optical system having the characteristics shown in the foregoing chart, the stop in the center of the relay lens is 0.077 millimeter from both the first and the second rod lenses. The second rod lens is a mirror image of the first rod lens. Thus, the second rod lens has a first surface with a radius of 8.0832 millimeters, a second surface with a radius of 5.4772 millimeters at a distance of 1.8 millimeters away, and a third, proximal-most surface with a radius of 8.0175 millimeters located 19.2 millimeters away. The indices of refraction and Abbe numbers of the elements of the second rod lens also correspond to those of the first rod lens. 
         [0052]    In the optical system having the characteristics shown in the foregoing chart, the second meniscus lens is located 0.1 millimeter from the proximal-most surface of the second rod lens. The distal-most surface of the second meniscus lens has a radius of 4.3062 millimeters and the proximal-most surface has a radius of 3.2446 millimeters. The second meniscus lens has a thickness of 2.4046 millimeters, an index of refraction of 1.788, and an Abbe number of 47.3. The image plane is located 0.6185 millimeter from the second meniscus lens. 
         [0053]      FIG. 3  shows a second embodiment in which the optical system  350  includes an objective lens (not shown), a relay lens  300  that includes two rod lenses  301  and  302 , and an eyepiece (not shown). The rod lenses  301  and  302  are doublets, in that they each include two simple lenses with a shared surface. Between the rod lenses  301 ,  302  is stop  307 . A first meniscus lens  305  is disposed between image plane  303  and rod lens  301  and a second meniscus lens  306  is disposed between image plane  304  and rod lens  302 . 
         [0054]      FIG. 3  shows each of the first and second meniscus lenses  305  and  306  are doublets comprising two simple lenses, as opposed to the single-lensed meniscus lenses of  FIG. 2 . First meniscus lens  305  comprises a plano-concave lens  305   a  and a plano-convex lens  305   b . These lenses are cemented together or otherwise joined in some embodiments, such that they have a proximal concave surface  305   c , a shared surface  305   d , and a distal convex surface  305   e . Likewise, second meniscus lens  306  comprises a plano-concave lens  306   a  and a plano-convex lens  306   b , with proximal convex surface  306   e , shared surface  306   d , and distal concave surface  306   c . The first and second meniscus lenses  305 ,  306  are substantially afocal lenses. They are designed so as to produce no substantial net convergence or divergence of collimated light. 
         [0055]    The first meniscus lens  305  and the second meniscus lens  306  can be identical doublet lenses, but oriented in opposite directions in the relay. As in the first embodiment shown in  FIG. 2 , the first and second meniscus lenses  305 ,  306  of the embodiment shown in  FIG. 3  are substantially afocal and configured to correct aberrations introduced into the system by the rod lenses, including astigmatism and field curvature. 
         [0056]    The ratio of the radii of the curved surfaces of the first and second meniscus lenses  305 ,  306  can be about 0.75 or 3:4. In other embodiments, this ratio can be different and can be varied alongside the other characteristics of the lens, such as its thickness, index of refraction, Abbe number, etc. This embodiment shows a design for the afocal meniscus elements including doublets. Numerous other optical designs are possible for use in the present invention, so long as they meet the general requirements of being afocal and a meniscus lens. The meniscus lenses according to the invention can be compound lenses that include more than two lenses, as well. 
         [0057]    In an embodiment similar to that shown in  FIG. 3 , the optical elements can have the following characteristics: 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Distance to 
                   
                   
                   
               
               
                   
                   
                 Next Surface 
                   
                   
                 Outer 
               
               
                   
                 Radius 
                 or Thickness 
                 Index of 
                 Abbe 
                 Diameter 
               
               
                 Surface 
                 (mm) 
                 (mm) 
                 Refraction 
                 Number 
                 (mm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Objective 
                 Infinity 
                 0.7354 
                 — 
                 — 
                 — 
               
               
                 First meniscus, concave 
                 −2.2414  
                 0.5 
                 1.846 
                 23.7 
                 1.8 
               
               
                 First meniscus, shared 
                 Infinity 
                 1.384 
                 1.788 
                 47.3 
                 2.79 
               
               
                 First meniscus, convex 
                 −2.9286  
                 0.1 
                 — 
                 — 
                 2.79 
               
               
                 First rod lens, proximal 
                 8.0175 
                 19.2 
                 1.603 
                 38.0 
                 2.79 
               
               
                 First rod lens, shared 
                 5.4772 
                 1.8 
                 1.547 
                 53.6 
                 2.79 
               
               
                 First Rod lens, distal 
                 −8.0832  
                 0.077 
                 — 
                 — 
                 2.79 
               
               
                 STOP 
                 Infinity 
                 0.077 
                 — 
                 — 
                 2.79 
               
               
                 Second rod lens, distal 
                 8.0832 
                 1.8 
                 1.547 
                 53.6 
                 2.79 
               
               
                 Second rod lens, shared 
                 −5.4772  
                 19.2 
                 1.603 
                 38.0 
                 2.79 
               
               
                 Second rod lens, proximal 
                 −8.0175  
                 0.1 
                 — 
                 — 
                 2.79 
               
               
                 Second meniscus, convex 
                 2.9286 
                 1.384 
                 1.788 
                 47.3 
                 2.79 
               
               
                 Second meniscus, shared 
                 Infinity 
                 0.5 
                 1.846 
                 23.7 
                 2.79 
               
               
                 Second meniscus, concave 
                 2.2414 
                 0.7354 
                 — 
                 — 
                 1.8 
               
               
                 Image 
                 Infinity 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
         [0058]    In the optical system having the characteristics shown in the foregoing chart, the objective is at the distal end of the relay lens and each successive surface is the next surface in the proximal direction until the image plane is reached. The distances and length measurements are in millimeters. 
         [0059]    The distal surface of the first meniscus lens is located 0.7354 millimeter from the objective of the relay. The distal-most, concave surface of the first meniscus lens has a radius of 2.2414 millimeters. The thickness of the plano-concave lens of the first meniscus lens is 0.5 millimeter. The plano-concave lens has an index of refraction of 1.846 and an Abbe number of 23.7. The thickness of the plano-convex lens of the first meniscus lens is 1.384 millimeters. The convex surface of the meniscus doublet has a radius of 2.9286 millimeters. The plano-convex lens has an index of refraction of 1.788 and an Abbe number of 47.3. 
         [0060]    The first meniscus lens is 0.1 millimeter from the distal surface of the first rod lens. The distal-most surface of the distal-most element of the first rod lens has a radius of 8.0175 millimeters and the element is 19.2 millimeters thick. The surface shared by the two elements of the rod lens has a radius of 5.4772 millimeters and is separated from the proximal-most surface of the rod lens by 1.8 millimeters. The proximal most surface of the first rod lens has a radius of 8.0832 millimeters. The distal-most element of the rod lens has an index of refraction of 1.603 and an Abbe number of 38.0, while the proximal-most element of the rod lens has an index of refraction of 1.547 and an Abbe number of 53.6. 
         [0061]    The stop in the center of the relay lens is 0.077 millimeter from both the first and second rod lenses. The second rod lens is a mirror image of the first rod lens. Thus, the second rod lens has a first surface with a radius of 8.0832 millimeters, a second surface with a radius of 5.4772 millimeters at a distance of 1.8 millimeters away, and a third, proximal-most surface with a radius of 8.0175 millimeters located 19.2 millimeters away. The indices of refraction and Abbe numbers of the elements of the second rod lens also correspond to those of the first rod lens. 
         [0062]    The second meniscus lens is located 0.1 millimeter from the proximal-most surface of the second rod lens. The distal-most, convex surface of the second meniscus lens, has a radius of 2.9286 millimeters. The plano-convex lens has a thickness of 1.384 millimeters, an index of refraction of 1.788, and an Abbe number of 47.3. The concave surface of the plano-concave lens has a radius of 2.2414 millimeters, and the plano-concave lens has a thickness of 0.5 millimeter, an index of refraction of 1.846, and an Abbe number of 23.7. The image plane is located 0.7354 millimeter from the second meniscus lens. 
         [0063]      FIG. 4  shows a third exemplary embodiment of the invention in which the optical system  450  includes an objective lens (not shown), relay lenses  400   a  and  400   b , and an eyepiece (not shown). Each of the relay lenses is substantially identical to the relay lens  200  shown in  FIG. 2 , except for the positioning of the afocal meniscus lenses. A first afocal meniscus lens  406   a  is disposed between image plane  404   a  and relay lens  400   a . A second afocal meniscus lens  405   b  is disposed between image plane  403   b  and relay lens  400   b . No afocal meniscus lenses are disposed between image plane  403   a  and relay lens  400   a  or between image plane  404   b  and relay lens  400   b . Images planes  403   b  and  404   a  may be the same image plane. The afocal meniscus lenses  406   a  and  405   b  may be substantially identical to the afocal meniscus lenses described in the embodiment shown in  FIG. 2 . Alternatively, the afocal meniscus lenses may be doublets, as described in the embodiment shown in  FIG. 3 . 
         [0064]      FIG. 5  shows an exemplary embodiment of an endoscope  510  with an optical system  550  that has an objective (not shown), three relay lenses  500   a ,  500   b ,  500   c , and an eyepiece  513 . Each of the relay lenses  500   a - c  includes two afocal meniscus lenses  505   a - c ,  506   a - c  disposed between the corresponding image planes  503   a - c ,  504   a - c  and the relay lenses  501   a - c ,  502   a - c . All of these optical elements and/or components are disposed in the shaft  508  of the endoscope  510 . Each of the relay lenses  500   a - c  in  FIG. 5  are substantially identical to the relay lens  200  shown in  FIG. 2 . Image planes  504   a  and  504   b , and image planes  504   b  and  503   c  may be substantially the same image plane. 
         [0065]    It is typical and well within the understanding of those of ordinary skill in the art that the endoscope  510  of  FIG. 5  includes additional optical elements for conditioning and transmitting the image from the distal end  511  to the proximal end  512  of the endoscope  510 . These often include, for example, a cover glass, objective lens, aperture stops, and field stops. Some embodiments include coupling elements for attaching the endoscope  510  to a camera or other device. 
         [0066]      FIG. 6  shows an exemplary embodiment of an endoscope  610  with an optical system  650  that has an objective lens (not shown), three relay lenses  600   a - c , and an eyepiece  613 . The endoscope  610  is similar to endoscope  510  in  FIG. 5  except that the afocal meniscus lenses associated with each relay lenses are doublets. Each of the relay lenses  600   a - c  in  FIG. 6  are substantially identical to the relay lens  300  shown in  FIG. 3 . 
         [0067]      FIG. 7  shows an exemplary embodiment of an endoscope  710  with an optical system  750  that has an objective lens (not shown), three relay lenses  700   a - c , and an eyepiece  713 . The endoscope  710  is similar to the endoscopes  510 ,  610  in  FIGS. 5 and 6 , respectively, except that the meniscus lenses associated with each relay lens are of different types. The distal-most relay lens  700   a  is substantially identical to relay lens  100  shown in  FIG. 1 , with no afocal meniscus lens. The intermediate relay lens  700   b  is substantially identical to relay lens  200  shown in  FIG. 2 , with an afocal meniscus lens. The proximal-most relay lens  700   c  is substantially identical to the relay lens  300  shown in  FIG. 3 , with a doublet afocal meniscus lens. 
         [0068]    It should be understood that the terms “about,” “substantially,” and like terms used herein when referring to a dimension or characteristic of a component indicate that the described dimension/characteristic is not a strict boundary or parameter and does not exclude variations therefrom that are functionally similar. At a minimum, such references that include a numerical parameter would include variations that, using mathematical and industrial principles accepted in the art (e.g., rounding, measurement or other systematic errors, manufacturing tolerances, etc.), would not vary the least significant digit. 
         [0069]    While several embodiments have been disclosed, it will be apparent to those of ordinary skill in the art that aspects of the present invention include many more embodiments and implementations. Accordingly, aspects of the present invention are not to be restricted except in light of the attached claims and their equivalents. It will also be apparent to those of ordinary skill in the art that variations and modifications can be made without departing from the true scope of the present disclosure. For example, in some instances, one or more features disclosed in connection with one embodiment can be used alone or in combination with one or more features of one or more other embodiments.