Zoom lens
CROSS-REFERENCE TO A RELATED APPLICATION 
     Reference is made to commonly assigned U.S. Pat. No. 4,757,372, entitled SLR ZOOM CAMERA. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to zoom taking lenses for photographic single lens reflex (SLR) cameras. 
     2. Description Relative to the Prior Art. 
     SLR cameras are known which include a camera body which supports photographic film in a film plane, a taking lens for imaging an object onto the film at the film plane, a shutter for controlling the flow of light to the film plane for determining the duration of the photographic exposure, and a viewfinder system for viewing the image which the lens would create on the film plane. The viewfinder system usually includes reflective means, in the form of a mirror, movable between a position in which it intercepts light rays which are directed by the taking lens towards the film plane and directs them laterally of the axis of the taking lens for viewing, and a position in which it is out of the way of the rays from the taking lens and allows those rays to approach the film plane. The first-mentioned position of the mirror may be termed its viewing position and the second-mentioned position may be termed its taking position. 
     In known camera and zoom lens combinations the mirror is behind the rearmost element of the lens. Such position of the mirror inevitably requires that the lens have a long back-focal-length. Additionally, the zooming motions must take place in front of the mirror. This restriction determines the first order properties of the system and usually results in large element diameters and/or lens length. 
     It is an object of the present invention to overcome the above restrictions. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a zoom taking lens for a photographic SLR camera having viewfinder means, the lens having a front to be towards an object to be photographed and an axis. The lens includes a first unit of lens elements at the front of the lens and a moving second unit of lens elements. The second unit of lens elements includes an air space between elements of the unit. There are axially movable reflective means disposed in the air space and inclined to the axis of the lens. The reflective means serve to direct light received from the object through the first unit and through at least one element of the second unit, to the viewfinder means. 
     The present invention also resides in a zoom taking lens for a photographic SLR camera having a short focal length less than the diagonal dimension of the image created on the film plane of the camera by the lens, the lens having a front to be towards an object to be photographed and an axis. The lens includes a negative first lens element unit at the front of the lens and a positive second lens element unit including a positive first sub-unit and a negative second sub-unit. The first and second sub-units are separated by an air space sufficiently large to accommodate reflective means for directing light received from the object through the first unit and through at least one element of the second unit, laterally to viewfinder means. The first lens element unit has an air space, or air spaces, the length, or the sum of the lengths, of which is greater than 10% of the focal length at the short focal length extreme of the lens. 
     The present invention also resides in zoom taking lenses having parameters and characteristics as variously defined in Tables 1 to 16 contained in the ensuing description and the drawings associated therewith. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagram of a first embodiment of a lens in accordance with the present invention, in a short focal length condition; 
     FIG. 2 is a diagram of the lens illustrated in FIG. 1 but in a long focal length condition; 
     FIG. 3 is an enlarged portion of FIG. 2 additionally showing reflective means, in a viewing condition, for directing rays into a viewfinder system; 
     FIG. 4 is a diagram of the lens represented in FIGS. 1 to 4 together with viewfinder means, in a short focal length condition; 
     FIG. 5 is a diagram similar to FIG. 4 but in an intermediate focal length condition; 
     FIG. 6 is a diagram similar to FIGS. 4 and 5, but in a long focal length condition; 
     FIG. 7 is a table giving some parameters of the taking lens illustrated in FIGS. 1 to 6; 
     FIG. 8 is a diagram illustrating a lens in accordance with a second embodiment of the present invention, in a short focal length condition; 
     FIG. 9 is a diagram of the lens illustrated in FIG. 8, in a long focal length condition; 
     FIG. 10 is a Table 3, giving some of the parameters of the lens illustrated in FIGS. 8 and 9; 
     FIG. 11 is a diagram of a lens in accordance with a third embodiment of the present invention, in a short focal length condition; 
     FIG. 12 is a diagram of the lens illustrated in FIG. 11, in a long focal length condition; 
     FIG. 13 is a Table, 5, giving some parameters of the lens illustrated in FIGS. 11 and 12; 
     FIG. 14 is a diagram illustrating a lens which is a fourth embodiment of the present invention, in a short focal length condition; 
     FIG. 15 is a diagram illustrating the lens illustrated in FIG. 14, in a long focal length condition; 
     FIG. 16 is a Table, 7, giving some parameters of the lens illustrated in FIGS. 14 and 15; 
     FIG. 17 is a diagram illustrating a lens which is a fifth embodiment of the present invention, in a short focal length condition; 
     FIG. 18 is a diagram of the lens illustrated in FIG. 17, but in a long focal length condition; 
     FIG. 19 is a Table, 9, of some parameters of the lens illustrated in FIGS. 17 and 18; 
     FIG. 20 is a Table, 11, similar to Tables 1, 3, 5, 7 and 9, but for a sixth embodiment of the invention; 
     FIG. 21 is a diagram illustrating a lens which is a seventh embodiment of the present invention, in a intermediate focal length condition; 
     FIG. 22 is a Table 13, giving some parameters of the lens illustrated in FIG. 21; 
     FIG. 23 is a diagram illustrating a lens which is an eighth embodiment of the present invention, in a short focal length condition; 
     FIG. 24 is a diagram of the lens illustrated in FIG. 23 but in a long focal length condition; and 
     FIG. 25 is a Table, 16, giving some constructional parameters of the lens illustrated in FIGS. 23 and 24. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The lens 20 represented in FIGS. 1 and 2 is a zoom lens intended for use in a photographic camera and has a focal length range of 29.3 to 116.0 mm. In FIG. 1 it is illustrated in the condition giving the former focal length and in FIG. 2 it is illustrated in the condition giving the latter focal length. 
     The lens 20 has an axis 21 and includes three units 22, 24, and 26 of lens elements. The unit 22 is at the front of the lens 20, that is, it is towards an object 27 to be photographed. The unit 22 moves both in zooming and for focussing. The unit 26 is at the rear of the lens and, in the present embodiment, does not move. The unit 24 is between the units 22 and 26 and moves in zooming. 
     The front first unit 22 is negative and includes three elements 28, 30 and 32. The element 28 is located on the object side of the lens (i.e. it is at the front of the lens) and is a negative meniscus. The element 30 is in the middle of the first unit and is a negative meniscus. The third element 32 is a positive meniscus. 
     The second unit 24 includes six elements 34, 36, 38, 40, 42 and 44 formed in two sub-units, i.e. a sub-unit of elements 34, 36 and 38 and a sub-unit of elements 40, 42 and 44. The elements 36 and 38 form a cemented doublet as do the elements 40 and 42. The element 34 is biconvex. The element 36 is biconvex and the element 38 is biconcave and together they form a meniscus. 
     The element 40 is biconvex and the element 42 is a negative meniscus and together they form a negative doublet. The element 44 is a negative meniscus. There is an air space 46 between the elements 38 and 40. The air space 46 has an axial length which is sufficiently large that the air space 46 can accommodate reflective means, in the form of a mirror 48, which serves a purpose similar to that performed by the movable mirror of a conventional SLR camera, namely to direct, to a viewfinder system, light rays from the object being photographed and which have passed through the taking lens. A SLR camera has the advantage that the eye of the photographer can see exactly the image that will be created on the film, for example, there is an avoidance of parallax problems, and the framing and focus condition are exactly presented to the photographer. In FIGS. 4 to 6, the viewfinder system is indicated by the reference numeral 49. In a conventional SLR camera the mirror is located between the rearmost element of the taking lens and the film plane. Such a position of the mirror requires that the taking lens has a large back focal length. A lens in accordance with the present invention, by providing a large air space between elements of the lens, allows the mirror to be disposed within the length of the lens and hence allows the zooming elements of the lens to be closer to the film plane than is possible in a conventional SLR camera and allows the insertion of a field flattening lens. This is apparent in FIGS. 4 to 6 which show the taking lens 20, mirror 48 and viewfinder system 49 in three different conditions including the extreme short focal length and maximum focal length positions. As can be seen in FIGS. 4 to 6, the mirror is movable along the axis of the lens in coordination with the corresponding movement of an adjacent lens element so that the mirror and the lens element occupy the same location during different stages of zooming. 
     FIG. 3 illustrates the front and second lens units 22 and 24 on an enlarged scale and in their positions for maximum focal length of the lens 20. The mirror 48 is shown supported on an arm 47 for pivotal movement about a pivot 50. The pivot 50 for the mirror 48 is integral with the mount (not shown) for the second lens unit 24 so that the mirror moves with the second lens unit during zooming. The mirror is shown in solid line in its viewing position and in broken line in its taking position. In its viewing position, the mirror 48 is disposed at an angle (45° in the present embodiment) such that the incident rays 45 arriving from the lens element 38 are reflected, as rays 43, laterally of the axis 21, out from between the lens elements 38 and 40 into the viewfinder system, which is not shown in FIG. 3 but is shown in FIGS. 4, 5 and 6. The viewfinder forms part of an SLR Zoom Camera which is described and claimed in U.S. Pat. No. 4,757,372, filed on the same day as the present application in the names of Ellis I. Betensky, Melvyn H. Kreitzer and Jacob Moskovich, the disclosure of which is specifically incorporated herein by reference. 
     The mirror 48 is of such a size that with all possible focal lengths of the lens, i.e. with all possible shapes of the bundles of rays 45 in the air space 46 between the lens elements 38 and 40, all of the rays 45 are intercepted by the mirror 48. The size of the mirror 48 and its inclination to the axis 21 determine the length of the air space 46 and when it is said in this specification that the air space is large it is meant that the air space is large enough to accommodate reflective means which are inclined to the axis and which intercept all light rays and which direct light received from the object through the first unit and through at least one element of the second unit, to viewfinder means. 
     The third lens unit 26, which is rearmost, contains a single lens element 52 and is stationary. The element 52 has overall convergent optical power, is biconvex and provides field flattening so that the image is planar and on the film plane 54. The air space between the units 24 and 26 is referenced 51 and is, of course, variable. 
     The first lens unit 22 is negative; the second lens unit 24 is positive; and the third lens unit 26 is positive. 
     The surfaces of the lens elements are referenced 1a through 18a. As is customary, contacting surfaces of a doublet are given the same reference numeral. 
     Various constructional parameters of the lens, when in its short focal length condition, are given in Table 1 which forms FIG. 7. All dimensions throughout this specification are in millimeters unless otherwise stated. 
     There is an aperture stop 53 located in near proximity to surface 11a. 
     The spaces between elements 32 and 34, i.e. air space 33, and between elements 44 and 52, i.e. the air space 51, vary during zooming and some values are: 
     
                       TABLE 2                                                     
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EFFECTIVE       AIR      AIR                                              
FOCAL           SPACE    SPACE                                            
LENGTH          33       51                                               
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29.30           43.34    0.25                                             
50.00           19.32    12.20                                            
85.00           5.91     32.49                                            
116.00          0.89     50.66                                            
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     It will be observed from Table 1 that the air space between surfaces 2a and 3a is 11.06 mm. and is greater than 10% of the short focal length extreme of the zoom lens, i.e. the 29.30 mm. focal length condition in Table 2. 
     In the above description of the first embodiment and in the drawings thereof, as elsewhere in this specification, the lens is understood to be focussed at infinity. 
     A lens 56 in accordance with a second embodiment of the present invention is illustrated in FIGS. 8 and 9. This lens 56 also includes three units 57, 59 and 61 with the front unit 57 moving for zooming and focussing and the second unit 59 moving for zooming. The third unit 61 is stationary. 
     The ten elements of the lens 56 are constructed identically in type and in formation into doublets and units, as the ten elements of the first embodiment and there will not be further description in this respect. 
     The lens includes a large air space 67 in the second unit which, in accordance with the present invention, is large enough, 23.5 mm., to accommodate reflective means as described above in relation to the first herein described embodiment. Various constructional parameters of the lens represented in FIGS. 8 and 9 are given in Table 3 in FIG. 10. 
     There is an aperture stop 55 which is located in near proximity to surface 7b. The aperture stop has a variable aperture. 
     There are air spaces 63 and 65 between the units 57 and 59 and between the units 59 and 61, respectively, which vary during zooming and the thicknesses of these air spaces 63 and 65 at various effective focal lengths are as follow: 
     
                       TABLE 4                                                     
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EFFECTIVE       AIR      AIR                                              
FOCAL           SPACE    SPACE                                            
LENGTH          63       65                                               
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29.30           43.28    0.25                                             
50.00           20.74    l2.10                                            
84.99           7.62     32.22                                            
115.99          2.70     50.20                                            
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     It will be understood from Table 3 that the air space between surfaces 2b and 3b is 10.32 mm. and is greater than 10% of the short focal length extreme of the zoom lens, i.e. the 29.30 focal length condition in Table 4. 
     A lens 58 in accordance with a third embodiment is illustrated in FIGS. 11 and 12 and some of its constructional parameters are included in Table 5 which constitutes FIG. 13. The lens 58 includes two units 60 and 64 of lens elements. The front unit 60 moves for zooming and for focussing and the second unit 64 moves for zooming. The second unit includes a large air space 62, in accordance with the present invention, which is large enough for receiving reflective means similar in purpose to the mirror 48 of the first above-described embodiment. The size of the air space is given in Table 6 below. 
     The first unit 60 of lens elements includes a positive meniscus element 301 at the front of the lens. The next element is a biconcave element 303 and the third element is a positive meniscus element 305. 
     The second unit 64 of elements includes eight elements arranged three ahead of the large air space 62 and five behind the air space 62. The first element of the second unit 63 is a biconvex element 307. The second and third elements 309 and 311 are a cemented meniscus doublet. The element 309 is biconvex and the element 311 is biconcave. After the air space 62 there is the fourth element 313 in the second unit, which is biconvex. The fifth and sixth elements 315 and 317 in the second unit are a cemented doublet. The element 315 is biconcave and the element 317 is biconvex. The seventh element 319 in the second unit, i.e. the fourth element after the air space 62, is a negative meniscus. The final element 321 in the second unit, which is at the back of the lens, is a negative meniscus. 
     In this third embodiment there is a variable aperture stop 66 located in near proximity to the surface 7c. Table 6 gives the length of the air space 62 for various effective focal lengths including the two focal length extremes. 
     
                       TABLE 6                                                     
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EFFECTIVE       AIR                                                       
FOCAL           SPACE                                                     
LENGTH          62                                                        
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29.30           36.03                                                     
50.00           16.53                                                     
85.01           5.17                                                      
101.02          2.60                                                      
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     It will be observed from Table 5 that the sum of the lengths of the air spaces between surfaces 2c and 3c and between surfaces 4c and 5c is 7.77 mm. and is greater than 10% of the short focal length extreme of the zoom lens, i.e. the 29.30 mm. focal length condition in Table 6. 
     A fourth embodiment of the invention is illustrated in FIGS. 14 and 15. Like the third embodiment, it includes two units 82 and 84 of lens elements both of which are movable for zooming and the front one of which is movable also for focussing. The air space between the two units 82 and 84 of lens elements is referenced 83 in FIGS. 14 and 15. There is a large air space 87, in accordance with the present invention, in the second lens unit 84 between surfaces 11d and 12d. 
     In the fourth embodiment there are eleven lens elements arranged in two units as in the third embodiment. The elements in the fourth embodiment are of the same types and sequence and are arranged in doublets and units in the same manner as the elements in the third embodiment and reference is made to the description of the third embodiment. 
     Some of the lens constructional parameters are given in Table 7 which constitutes FIG. 16. The length of the air space at various focal lengths, including the two focal length extremes, is given in Table 8 below. There is a variable aperture stop 86 which is in near proximity to the surface 11d. 
     
                       TABLE 8                                                     
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EFFECTIVE       AIR                                                       
FOCAL           SPACE                                                     
LENGTH          83                                                        
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29.30           45.14                                                     
50.00           20.51                                                     
85.01           6.16                                                      
116.01          0.68                                                      
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     It will be observed from Table 7 that the sum of the lengths of the air spaces between surfaces 2d and 3d and between surfaces 4d and 5d is 10.22 mm. and is greater than 10% of the short focal length extreme of the zoom lens, i.e. the 29.30 mm. focal length condition in Table 8. 
     A lens in accordance with a fifth embodiment of the invention is illustrated in FIGS. 17 and 18. Like the third and fourth embodiments, this fifth embodiment includes two lens units 100 and 102, both of which are movable for zooming and the front unit 100 of which is movable also for focussing. The fifth embodiment includes a large air space 108 in the second unit 102 of lens elements. In this embodiment, the large air space 108, intended to accommodate the reflective means, does not have a fixed dimension but like the air space 106 between the units 100 and 102, it is variable in length. There is a variable aperture stop 104 which is in near proximity to the surface 11e. 
     The eleven lens elements in the fifth embodiment are of the same type and are in the same sequence as the eleven lens elements in each of the third and fourth embodiments and they are formed into doublets and are arranged in units in the same manner as in the third and fourth embodiments, and reference is made to the descriptions of the third and fourth embodiments. 
     Some of the constructional parameters of the lens illustrated in FIGS. 17 and 18 are given in Table 9 which forms FIG. 19. The lengths of the two air spaces 106 and 108 are given in the following Table 10 for various focal lengths, including the two focal length extremes. 
     
                       TABLE 10                                                    
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EFFECTIVE       AIR      AIR                                              
FOCAL           SPACE    SPACE                                            
LENGTH          106      108                                              
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29.30           46.22    24.14                                            
50.00           21.30    23.11                                            
85.00           6.38     22.53                                            
116.00          0.68     22.20                                            
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     It will be observed from Table 9 that the sum of the lengths of the air spaces between the surfaces 2e and 3e and between surfaces 4e and 5e is 12.43 mm. and is greater than 10% of the short focal length extreme of the zoom lens, i.e. the 29.30 mm. focal length condition in Table 10. 
     In that the large air space in the second unit 102 of lens elements is variable in length, it is to be understood that even in the condition of the lens which makes the length of the air space 106 the shortest, which is the extreme long focal length condition in the present embodiment, there is space enough for the reflective means. Thus, the present invention is to be understood as including lenses in which the air space for accommodating the reflective means is variable in length, but which in its shortest length is long enough to accommodate reflective means. 
     A sixth embodiment of the present invention is generally similar in structure to the first embodiment described above with reference to FIGS. 1 to 7, but has different surface radii and other parameters. The surface radii, element and air space (designated 33&#39; and 51&#39; to correspond to spaces 33 and 51 in the first described embodiment) thicknesses and element materials are given in Table 11 which is FIG. 20. The lengths of the variable air spaces 33&#39; and 51&#39;, at various effective focal lengths of the lens are as follows: 
     
                       TABLE 12                                                    
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EFFECTIVE       AIR      AIR                                              
FOCAL           SPACE    SPACE                                            
LENGTH          33&#39;      51&#39;                                              
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29.30           41.67    0.25                                             
50.00           19.01    12.15                                            
85.00           5.84     32.42                                            
116.02          0.89     50.48                                            
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     It will be observed from Table 11 that the air space between surfaces 2f and 3f is 10.91 mm. long and is greater than 10% of the short focal length extreme of the zoom lens, i.e. the 29.30 mm. focal length condition in Table 12. 
     In this sixth embodiment, the aperture stop, similar to aperture stop 53, is located in near proximity to the surface 11f corresponding to surface 11a. 
     It will be observed from a review of Tables 1 and 11 that the large air space for accommodating the reflective means is 2 mm. longer in the sixth embodiment than in the first embodiment which is advantageous as providing additional space for the viewfinder system, such as system 49 in FIGS. 4, 5 and 6. 
     A seventh embodiment of the present invention is represented in FIG. 21 which shows a lens 202 which includes a first lens element unit 204 at the front of the lens, a second lens element unit 206 and a field flattening element 208. 
     The first lens unit 204 includes two elements 210 and 212. The front element 210 is a negative meniscus. The second lens element 212 in the first lens unit is a positive meniscus. There is a variable air pace 216 between the first and second units 204 and 206. The second unit 206 includes four lens elements 218, 220, 222 and 224. The front element 218 of the second unit is biconvex. The second and third elements 220 and 222 are formed as a doublet and are, respectively, biconvex and a negative meniscus. The fourth element 224 of the second unit is negative meniscus. There is a large air gap 226, in accordance with the present invention, between the third and fourth elements of the second unit 206. 
     The field flattening element 208 is separated form the second unit 206 by a variable air space referenced 228 in FIG. 21. 
     The second unit 206 moves for zoomimg and the first element 204 moves for zooming and focus compensation. 
     Table 13, which forms FIG. 22 gives the radii of the surfaces 1g to 13g of the elements of the lens 202; the thicknesses of the air spaces; characteristics of the materials of the elements; and the clear apertures of the elements. Table 13, which follows gives the lengths of the air spaces for various effective focal lengths, including the two focal length extremes, of the lens. 
     
                       TABLE 14                                                    
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EFFECTIVE       AIR      AIR                                              
FOCAL           SPACE    SPACE                                            
LENGTH          216      228                                              
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29.30           38.34    0.25                                             
50.00           17.51    12.86                                            
85.00           5.44     32.33                                            
116.01          0.89     53.27                                            
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     There is a variable aperture stop 230 located in near proximity to surface 9g. 
     It will be observed from Table 13 that the air space between surfaces 2g and 3g, i.e. the air space between the two elements 210 and 212 of the first unit 204, is 10.56 mm. which is more than 10% of the short focal length extreme of the zoom lens, i.e. the 29.30 mm. focal length in Table 13. 
     An eighth embodiment of the present invention is illustrated in FIGS. 23 and 24. The lens 300 illustrated in FIGS. 23 and 24 includes a first unit 302 at the front of the lens, a second unit 304 and a third unit 306. The first unit 302 is moveable for zooming and focussing, the second unit 304 is moveable for zooming and the third unit 306 is fixed. 
     The first unit 302 includes a first element 308 which is a positive meniscus, a second element 310 which is a negative meniscus, and a third element 312 which is a positive meniscus. 
     The second unit 304 includes a biconvex element 314 and a doublet formed of a biconvex element 316 and a negative meniscus element 318. The elements 314, 316 and 318 form a first sub-unit which is separated from a second sub-unit by a large air space, in accordance with the present invention. The second sub-unit includes an element 320 which is a positive meniscus and an element 322 which is a negative meniscus. 
     The third unit 306 includes a single element 324 which is biconvex and flattens the field. 
     There is a variable air space 326 between the first and second units and a variable air space 328 between the second and third units. The lengths of the variable air spaces 326 and 328 are given in Table 15 below. 
     
                       TABLE 15                                                    
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EFFECTIVE       AIR      AIR                                              
FOCAL           SPACE    SPACE                                            
LENGTH          326      328                                              
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29.29           41.11    0.25                                             
49.99           18.75    13.08                                            
85.01           5.77     34.91                                            
116.06          0.90     54.29                                            
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     There is a variable aperture stop 330 in near proximity behind the element 318 of the first sub-unit of the second unit. 
     Various constructional parameters of the lens 300, when in short focal length condition, are given in Table 16 which forms FIG. 25. 
     It will be understood from Table 16 that the air space between surfaces 2h and 3h is 10.57 mm. long and is greater than 10% of the short focal length extreme, i.e. the 29.29 mm. focal length condition in Table 15. 
     It will be observed from the ensueing Table 17 that the ratio of the absolute value of the focal length of the first unit to the focal length of the second unit, lies within the range of 1.4:1 to 2.8:1, for all of the embodiments herein described. 
     
                       TABLE 17                                                    
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EMBODI- ABSOLUTE VALUE  FOCAL                                             
MENT    OF FOCAL LENGTH LENGTH                                            
FIGS.   OF lst UNIT     OF 2nd UNIT RATIO                                 
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1-7     57.442          30.716      1.87:1                                
 8-10   57.452          30.261       1.9:1                                
11-13   50.123          27.525      1.82:1                                
14-16   57.082          30.526      1.87:1                                
17-19   60.002          40.287      1.49:1                                
20      57.468          30.421      1.89:1                                
21,22   57.819          29.999      1.93:1                                
23-25   57.542          31.344      1.84:1                                
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     It will be observed that all of the described embodiments of the invention have a short focal length which is less than the diagonal measurement, 43.27 mm., of a 35 mm. photographic image. 
     In the embodiments of the invention described above, all of the lens elements are spherical. It is to be understood that one or more aspheric surfaces may be used in embodiments of the present invention and the invention is to be understood as including embodiments in which an aspheric surface or aspheric surfaces are used. 
     While the reflective means, in the form of mirror 48, has been illustrated only in FIGS. 3 to 6, which are some of the Figures illustrating the first embodiment of the present invention herein described, it is to be understood that the other embodiments herein described also have reflective means in the large air space in the second lens element unit, when intended for use with SLR cameras. The reflective means are omitted from all Figures other than FIGS. 3 to 6 to avoid overcrowding the Figures and thereby aid understanding. The lenses herein described but without reflective means in the large air space in the second lens element unit are regarded as part of the present invention. 
     Reflective means other than the mirror arrangement described with reference to and illustrated in FIGS. 3 to 6 may be used. For example, mirrors movable into and out of the air space in manners other than that specifically illustrated may be adopted. Also, reflective means which are permanently in the air space may be used, such as half silvered mirrors and other forms of beam splitters. 
     The invention has been described in detail with particular reference to presently preferred embodiments, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.