US2685230A - Large aperture telephoto objective - Google Patents

Description

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LARGE APERTURE TELEPHOTO OBJECTIVE 3 Sheets-Sheet l T Filed Nov. 6, 1947 BY M45 A Troie/v6# Aug 3, 1954 J. G. BAKER LARGE APERTURE TELEPHoTo OBJECTIVE 3 Sheets-Sheet 2 Filed NOV. 6. 1947 INVENTOR. .A4/5 34a/fe lBY m ATTOQ/VEY t ,Mawr- Aug. 3, 1954 J. G. BAKER LARGE APERTURE TELEPHoTo OBJECTIVE 3 Sheets-Sheet 3 Filed NOV. 6, 1947 Mae.,

Patented Aug. 3, 1954 LARGE APERTURE TELEPHOTO OBJECTIVE James G. Baker, Waban, Mass., assignor to the United States of America as represented by the Secretary of War Application November 6, 1947, Serial No. 784,478

8 Claims. l

This invention relates to large aperture photographic objectives and particularly to telephoto objective lens systems.

It is a primary object of this invention to provide a telephoto objective having a large aperture and long equivalent focal length.

It is another important object of this invention to provide a lens system for telephoto objectives having an air spaced triplet using a high index flint negative element between two low index positive crown elements in the objective component and having air spaced flint positive elements preceding negative crown elements in the rear component to eliminate distortion without impairing definition at maximum telephoto effect on a flat eld.

It is a further object of this invention to provide a compactglens system for telephoto objectives of large aperture and long equivalent focal length by reecting the light beam on mirrored surfaces.

There has been an expressed need for long equivalent focal length telephoto objectives with a low telephoto ratio. The telephoto ratio as used in this disclosure refers to the ratio of the camera length as measured between the focal plane and the vertex of the front lens surface, and the equivalent focal length. Such telephoto objectives have innumerable uses and are particularly desirable for aerial photography Where a high degree of resolution and contrast are necessary for precision reconnaissance, and the like. In usual practice, lens forms that have been found to perform satisfactorily in small focal lengths have been scaled up and modied for use in aerial photography at large focal lengths. However, this practice has been found to lead to photographs of lowered linear resolution and lowered contrast that in part defeat the purpose of using large focal lengths. Lenses of large focal length must therefore employ improvements in design beyond the requirements of scaling if satisfactory results are to be achieved. At the same time in order to avoid cumbersome and weighty construction of large lens systems it has proved advisable to make use of the telephoto type of optical system, a choice that makes the burden on the optical design doubly difficult. In the present invention a type of telephoto system has been devised that obviates all the above-mentioned difficulties and provides satisfactory results at large focal lengths.

The present invention provides a lens system for large aperture telephoto objectives which produce good resolution and definition at maximum telephoto effect on a fiat field that is distortionless and is substantially free of undesirable aberrations. This is accomplished by using an airspaced triplet consisting of a high index flint negative element between two low index crown positive elements for the forward objective component and air spacing one or both of two doublets in the rear component in which a positive flint element may precede a negative crown element in each doublet. The use of the low index crown and a high index int in the forward objective component, contrary to the usual practice in standard lenses, favors the telephoto effeet, permits distortion correction and provides the elimination of zonal terms in the sine condition. However, the above elements tend to increase lateral color which is restricted by the lter and by the restricted angular coverage of these large lenses.

In the construction of the lens system, placing a filter in the converging beam behind the last element is contemplated. The aberrations of the filter are small and can be taken into account in the lens design. Positioning the filter behind the lens system has several advantages in that the surface gure need be only a fraction as good as a lter in front of the lens system; the lter diameter need be only about half as large which simplifies manufacture; and the filter thickness can be used to adjust the best focus in any color that goes with the filter, that is, an infra-red filter should be thinner than the yellow filter in order that the infra-red focus lie on the focal plane determined for yellow light.

Referring to the drawings:

Fig. 1 shows a telephoto objective lens system in accordance with the invention Fig. 2 shows another embodiment of the invention; and

Fig. 3 shows a modification of the lens system shown in Fig. l with the ray path reflected on mirrored surfaces.

Referring to Fig. 1, which illustrates the invention, there is shown by way ofl example a telephoto objective lens system that consists of lenses I to VII, inclusive, having radii of curvature represented by R1 to R16, inclusive; lens thicknesses represented by D1 to Da, inclusive; and air spaces represented by S1 to Ss, inclusive. The lenses I, II, and III are on the side of the longer conjugate and will be referred to as the forward objective component; while the lenses IV to VII, inclusive, are on the side of the shorter conjugate and will be referred to as the rear component.

The element VIII is a filter of 'plano-plano construction that is placed in the converging beam of the last element with respect to the direction of light travel through the system. The index of refraction and thickness of this filter must be considered in the lens system and is 3 given in the examples of Fig. 1 although the iilter may be changed for diiierent light.

Two examples of specifications are given below in accordance with the invention. Example 1 provides a lens system of iO-inch equivalent focal length and a relative aperture of f/5. The notation R1, Ra etc. refers to the successive radii of curvature, counting from the front, which will be denoted as convex toward the longer conjugate in the absence of any sign, and will be denoted by a minus sign when the radii of curvature is concave toward the same, as is conventional. Nn is used in the examples to denote the refractive indices of the lens elements for the D-line and the Abbe-V number is used to denote the dispersion value. The radii, thicknesses and spacing of the lens elements for these two examples, and all other examples in this disclosure, are given in inches. The refractive indices and Abbe-V numbers of Example 2 are the same as for Example 1. As may be noted in Example 2 the lens thicknesses and radii of 5 curvature as well as the air spaces of Example 1 10 duced in practicing the invention of Example 1.

By scaling up or down the lens thicknesses, radii of curvature, and air spaces, and by altering these quantities by small amounts according to the requirements of optical design, one can produce 15 telephoto objective lens systems of various focal lengths, relative apertures, and angular coverages without departing from the spirit and scope of this invention.

Example 1 [F=40 inches i/] Thickness S ace Abbo-V El Rad (inches) (inches) (inrhes) Nd Number R1 Il 14. 058 I D11'. 882 1. 5168 64. 5

Sl- .264 Rx 33.942 II Dif-.405 1.7195 29.3

Sg- .051 Rl l 7. 664 m Dx". 941 1. 517 64. 5

Re l 19. 722

S;- 9.454 R1 Il 40.066 IV D4-. 395 1. 6164 36. 6

Sul .819 Rg Il 5.724 v DIU. 356 1. 6102 58.8

Rxo- 14.14

Sl- 3.002 R11- 11.254 VI Dlll. 499 1. 6164 36.

Rn'- 17.347 VII DFI-407 1. 6102 58.8

B1- .916 R15-Plano VIII. Dn. 523 1. 517 Y7, R-2

Ril-P18110 Example 8 [F- inch f/5] E1. man (mene.) 'lfxlgg (gigs) N. Abbo-v Number R1 Il 2,1. 525 I D11-1.351 1. 5168 64.5.

Sx- .401 Ra l 51.436 II Dz- .614 1. 7195 29.3.

Sa- .077 R5 11.649 m Dl*1 144 1. 517 64.5.

5;-14. 677 R1 l 60. 73 Iv D4- .599 1.6164 36.6.

S4- 1.241 R '-8. 670 v D6* .542 1.6102 58.8.

B5- 4. 550 R11- 17.704 VI. D;- 732 1. 6164 36.6.

Sg# 1.288 R15-l 32. 374 VII. D11-'1. 573 1. 6102 58.8.

' B1- 1. 418 Ril-Plano vm. Dl- 541 l. 517 Grange Film.

It may be noted that the element D: in Example 2 has a. large central thickness that is not ordinarily necessary for the optical requirements ot the 60inch, f/5 telephoto objective. This great thickness is introduced into the design of this telephoto objective for the purpose of particularly adapting this optical system for high altitude ilying in which the optical system is evacuated. The element Dn forms one boundary of the evacuated lens system and is subjected to great pressure which pressure depends on altitude. The other boundary of the vacuum space containing the optical system is formed, in actual practice, by a plane-parallel window of considerable thickness but of no optical effects and therefore has not been shown in the optical system.

Referring to Fig. 2 there is shown a modification o! the lens system of Fig. 1 which is a abl infra-red tele hoto use. Reference is rected to e escriptive c ar below in which Referring to Fig. 3 there is shown a modication of the lens system of Fig. 1 in which two front surface mirrors M1 and Mz are used to direct the light rays in a U-shaped path. In this manner the camera in which this lens system may be used can be made more compact. A full description of this lens system is given below. The descriptive example given in this chart will provide a 60-inch f/6 telephoto objective but the specifications of lens thicknesses, radii of curva.- ture, and air spaces may be scaled up or down for other focal lengths and relative apertures, if desired, without departing from the spirit and scope of this invention.

[F=60lnches f/] Th1 kms Distance Abb6v El. Radiianches) (memes) (Inches) N5 Number S1- .261 C.- -47.96s B.. t.- .744 1.72 29.3.

Sl- .061 0|- 11.456 o ti- .954 1.517 64.5.

S|==14.451 C1 59.524 D n- .695 1.617 36.6

s.- 1.122 -ov- 9.196 E -zi- .595 1 6106s 57.2

S5- 6.977 0n- 18.572 F-.. t.- .695 1.617 36.6.

S5- .726 c11- -25819 i G 11- .601 151065 57.2.

s1- .595 C15-'Plano H t.- .496 1.511 R, or orange ciu-Plano Filter.

[F-io mem 5] The rear element of the rear component of the 60 modications shown in Figs. 2 and 3 could be Thickn s s am Abbey increased in thickness to adapt these optical sys- El. RadiiClncheS) (Inche (111,511.18) Nd bigltems for evacuated high altitude systems by altering the rear component in accordance with r 13 387 optical design, if desirable. 1 d,=,g66 1 5168 6.1 5 65 Many advantages have been attained by the r,=141.6a s a 261 above described telephoto objectives over other f, 34 003 known systems. The low optical power and form 2 r 34 274 d1=-396 1-7195 29-3 of the rear component make these telephoto s,= .05o objective systems suitable for incorporating tem- 3 fl 7-54 d z 924 1 5164 64 5 70. perature and air density compensation and the 19,699 i' rear elements of each is particularly suitable for r 3884 SF 9'317 acting as one boundary of an evacuated lens 4 1 d=.3s7 1.6192 58.8 system, illustrated in Fig. 1, Example 2. The fl* -58-074 Sw 810 `choice of glass types and lens forms brought about fl l 5.580 75 the fact that the negative rear component o! four lenses lies at an unusually low height in the converging beam making possible the use of a. relatively small iris diaphragm and a betweenthe-lens shutter of useful size for camera opera.- tion. These telephoto objective systems each provide very good resolving power and permit faster shutter speeds which make them extremely useful for aerial photography.

The 60-inch f/6 telephoto objective lens system, using mirrors to direct the rays in a reverse direc- [F-io inches. i/s] Thickness Space Abbe-V El' Rad (Inches) (inches) (Inches) Nd Number R1 14.058 I Di=. 882 l. 5168 64. 5

Rz l'-115. 80

S1= 264 Ri 33. 942 II D1=. 405 1. 7195 29. 3

Si: 051 Rs 7. 664 III Da=. 941 1. 517 64. 5

S3=9. 454 R1 40. 066 IV D4=. 395 1. 6164 36. 6

S= 819 Re 5. 724 V D5=. 356 1. 6102 58. B

Rio= 14. 164

S5=3. 002 Ru= 11. 254 VI D=l. 499 1. 6164 36. 6

Riz= 37. 075

Sa= 850 Ria= 17. 347 VII :Dr-I. 407 1. 6102 58. 8

S1 916 R15=Plano VIII Dg-I. 523 1. 517 Y7, R-2

R|=P1ano f Sa=1l. 631 l tion, has the advantage of providing a compact camera, particularly for aircraft where space is usually at a premium, without sacrificing any of the advantages of the systems of Fig. 1 and 2. AWhile preferred forms of the invention have been shown and described, it is to be understood 2. A telephoto objective having numerical data 0 as set forth in the following chart where R1, Rz,

that modmcations and changes may be made 4.-, air separations, Nd represents the indices of rewithout departing from the spirit and scope of this invention and I desire to be limited only by the scope of the appended claims.

fraction, the Abbe-V number represents the dispersion values, and F represents the equivalent focal length:

Thickness Space Abbe-V El. Radi! (Inches) (Inches) (Inches) N .i Number R1 l 21.525 L Di=1.351 1.5168 64.5.

Si .401 Rl l 5L436 II Dz- .614 1.7195 29.3.

Si .077 R; =l 11.649 III DPI. 144 1. 517 64.5.

Rc Il 30.121

Sa=14.677 R1 I 60.72 IV D4== .599 1.6164 36.6.

S4- 1.241 R- 8. 670 V D|== .542 1.6102 58.8.

Rin* 21.442

Sl. 4.550 Ru- 17.704 VI Dl* .732 1. 6164 36.6.

So- 1.288 Rn- 32.374 VII D1=1.573 1.6102 58.8.

S11- 1. 418 Ris-Plano VIII Di- .541 1.517 Orange Filter.

RuPIBllO represent the axial 5. A large aperture telephoto objective lens system comprising; a convergent front objective component having a focal point on the side of the shorter conjugate and having the front and of the elements, s1. s2, represent the axial 5 rear surfaces thereof convex toward the longer air separations, Nd represents the indices of reconjugate to produce a, second principal plane fraction, the Abbe-V number represents the disforwardly of the center ofA said objective compersion values, and F represents the equivalent ponent; and a rear divergent eOmDOrlen Speeed focal length: on the side of the shorter conjugate of said front [hm inches 5] 10 objective component not less than 1A; or more than EA the front objective component focal Abb V length and having a focal point on the side of the Thickness space f longer conjugate in front of said front objective E1. R d1 In 11 N N a I c es) (Inches) (Inches) d le? component a distance not more than 4 or less than 2 times the axial distance along the optin 13.367 cal axis between the two components, said rear 1 r, 141'63 d* :'866 1'5168 64'5 divergent component including at least two 34 003 s1= .261 doublets on the side of the front convergent 2 f (1,: 393 1,7195 29.3 component each having a positive element pren 34.274 s o ceding a negative element in the order named ,5: 7.54 from said front objective component and being r 19 699 d1 =.924 1.6164 64.5 spaced apart a distance not less than 1/4 or more s.: 9 317 than 1/2 the axial distance along the optical axis 4 f1 3&84 d 387 l 6102 58 8 between the two said components, one element n: 58614 25 of each doublet being flint and the other ele- 5 580 S= -810 ment of each doublet being crown, the crown ele- 5 d5 149 1,6164 36,6 ments having the same index of refraction and f1= 134309 Si: 3 162 the same dispersive value and the flint elements r= 16.91 having the same index of refraction and the same 6 m: 23 227 d =513 1'6164 3616 30 dispersive value, the index of refraction of the 7 d1 =.15o 1.6102 58.8 last-mentioned flint elements being greater than "l: 35-941 S6: 513 1.61 with a dispersion value of less than 40 and r11= 17. 772 the index of refraction of the crown elements be- 8 m: Plano d =-150 1-6102 58-8 ing less than 1.61 with a dispersion value greater 9 d, 513 1.6164 36.5 35 than 55, the front surface of each doublet and the m: 4s-873 10:.150 1.61908 6M rear surface of the rearmost doublet being con- 1o m= 37.189 vex toward the longer conjugate to establish in 51:12' 746 said rear divergent component together with said l front objective component a second principal 4. A telephoto obiectivehavmg numerical data 4o piane for the lens system forward of said front aS Set forth in the fOllOWlng Chart Where C1, C2, objective component and a focal plane for the represent the radii Of eurvetllre 0f the Sllrlens system rearwardly of the rear divergent feces. t1. t2, represent the axial thicknesses component such that the axial distance along of the elements, 11.12, represent the axial air the optical axis from the front vertex of the 0bseparations, N5 represents the indices of refrac- 45 jective component to said focal plane of the lens tion, the Abbe-V number represents the dispersystem is less than 82% of the distance from said sion values, and F represents the equivalent focal second principal plane of the lens system to said length: focal plane of said lens system, and the distance [F= inches f/] El Radll Thickness Distance Nd Abbe-V (Inches) (Inches) (Inches) Number cl 24. 521 A t1=1. 055 1. 517 64.5.

s1= .261 C3 47.038 B 1= .744 1.72 29.3.

s== .061 c1= 11.456 c 11= .954 1.517 64.5.

s1=14 451 o1= 59.524 D t= .695 1. 617 36.6.

s.- 1.122 c5= 9.196 E t1= .695 1.61669 57.2.

s.- .726 C11= 25. 819 G t1- .601 1 6106s 57.2.

clp 63.227 C Plano S1=l .595 H l .496 1.517 R1 61- orange C15=P11m6 Futer.

from the front vertex of said front objective component to the focal plane of the lens system is not more than 3 or less than 2 times the distance from the rearmost vertex of the rear divergent component to the focal plane of the lens system to provide a large aperture telephoto objective of relative long equivalent focal length that is distortionless and substantially free from undesirable aberrations to produce good resolution and denition on a flat field.

6. A large aperture telephoto objective lens system as set forth in claim 5 wherein the forward doublet of said rear divergent component has the positive and negative elements thereof air spaced an axial distance along the optical axis not less than 1/15 or greater than 116 the axial distance along the optical axis between said two components and forming a centrally thick air lens therebetween.

'7. =A large aperture telephoto objective lens u system comprising; a convergent front objective f members on the side of the triplet component the triple component consisting of a negative high index dint element coaxially air spaced between two positive low index crown elements, the foremost air space characterizing a centrally thick meniscus air lens concave toward the longer conjugate and the rearmost air space characterizing a centrally thin meniscus air lens convex toward the longer conjugate, said two low index crown elements having the same index of refraction and the same dispersion value, the index of refraction thereof being lower than 1.52 with the dispersion value thereof being greater than 60 and the index of refraction of said high index flint element being greater than 1.71 with a dispersion value less than 30, the powers of said positive crown elements being greater than the power of said negative flint element to produce its focus on the optical axis thereof on the side of the shorter conjugate, and the front and rear surfaces of said triplet component being convex toward the longer conjugate to establish a second principal plane forwardly of the center of said triplet component; and a divergent rear cornponent spaced along'the optical axis on the side of the shorter conjugate of said triplet component not more than 1%, or less than V3 of the focal length ofwsavd triplet component,|said divergent `."'rea'r componitcl'tdmg east two doublet doublet members of the divergent rear components each having a positive element preceding a negative element in the order named from the longer conjugate with one element of each doublet being ilint and one element of each doublet I being crown, the crown elements having the same index of refraction and the same dispersion value and the iiint elements having the same index of i refraction and the same dispersion value, the index of refraction of the last mentioned iiint elements being greater than 1.61 with a dispersion value l2 ,less than and the index of refraction of the fcrcwn elements being less than 1.61 with a disfpersive value greater than 55, said doublets being separated by more than f4 and less than 1/2 the axial distance along the optical axis between the said triplet component and said rear divergent component, the front surface of the forward doublet and the front and rear surface of the rearward doublet being convex toward the longer con- "jugate to produce in combination in said rear l,divergent component a second principal plane #therefor forward of the center of said rear divergent component and to produce a focal point on the side of the longer conjugate forward of said triplet component not more than 4 or less than 2 times the distance of said triplet component from said rear divergent component to establish in the combination of said rear divergent :component and said triplet component a second principal plane forward of said triplet component and a focal plane rearwardly of said rear divergent component, the axial distance along the optical axis from the front vertex of said triplet component to said focal plane of said lens system being not less than 2 or more than 3 times the axial distance along the optical axis from the rear vertex of said rear divergent component to said focal plane of the lens system which constitutes the telephoto effect of the lens system, and the axial distance along the optical axis from the front vertex of said objective triplet component to said focal plane of said lens system being not more than 82% of the distance from said second principal plane of the lens system to said focal plane of the lens system constituting the telephoto ratio of the lens system whereby to provide a large aperture telephoto objective of long equivikalent focal length.

8. A large aperture telephoto objective lens system as set forth in claim 7 wherein the foremost doublet of said rear divergent component has the positive and negative elements thereof air spaced an axial distance along the optical axis not less than 1,(,5 or greater than 116 the axial distance along the optical axis between said two components, said air space forming a centrally thick air lens.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,791,276 Konig Feb. 3, 1931 2,378,170 Aklin June 12, 1945 FOREIGN PATENTS Number Country Date 4,523 Great Britain of 1906 7,229 Great Britain of 1913 222,709 Great Britain Oct. 9, 1924 471,565 Germany Feb. 15, 1929 388,215 Great Britain Feb. 2'3, 1933

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