the invention will now be described in reference to fig2 - 7 , which show exemplary embodiments of the present invention . fig2 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 fig2 , 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 fig2 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 . in the embodiment of fig2 , 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 . in the embodiment of fig2 , 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 . in some embodiments , including the embodiment shown in fig2 , 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 . 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 . in an embodiment similar to that shown in fig2 , the elements of the optical system can have the following characteristics : 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 . 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 . 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 . 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 . 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 . fig3 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 . fig3 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 fig2 . 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 . 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 fig2 , the first and second meniscus lenses 305 , 306 of the embodiment shown in fig3 are substantially afocal and configured to correct aberrations introduced into the system by the rod lenses , including astigmatism and field curvature . 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 . in an embodiment similar to that shown in fig3 , the optical elements can have the following characteristics : 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 . 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 . 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 . 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 . 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 . fig4 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 fig2 , 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 fig2 . alternatively , the afocal meniscus lenses may be doublets , as described in the embodiment shown in fig3 . fig5 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 fig5 are substantially identical to the relay lens 200 shown in fig2 . image planes 504 a and 504 b , and image planes 504 b and 503 c may be substantially the same image plane . it is typical and well within the understanding of those of ordinary skill in the art that the endoscope 510 of fig5 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 . fig6 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 fig5 except that the afocal meniscus lenses associated with each relay lenses are doublets . each of the relay lenses 600 a - c in fig6 are substantially identical to the relay lens 300 shown in fig3 . fig7 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 fig5 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 fig1 , with no afocal meniscus lens . the intermediate relay lens 700 b is substantially identical to relay lens 200 shown in fig2 , with an afocal meniscus lens . the proximal - most relay lens 700 c is substantially identical to the relay lens 300 shown in fig3 , with a doublet afocal meniscus lens . 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 . 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 .
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 .