WHOLE SYSTEM ZOOM AND VARIFOCAL LENS WITH INTERMEDIATE IMAGE

1. Field of the Invention.

This disclosure relates to optical lenses and more particularly to such lenses that effect intermediate images with simplified optical design while maintaining improved vignetting and performance.

2. Description of the Prior Art.

Previous optical lens systems with intermediate images (e.g., U.S. Patent Nos. 6,961,188, 7,009,765, and others) have been designed with too much emphasis on separating the optical system into two systems; the front group between the object and intermediate image and the rear group between the intermediate image and the final image. This method adds cost and complexity by putting constraints on the designer to correct optical aberrations such as focus, color, and thermal performance in each group separately. Also vignetting and stop position advantages are not fully realized. Accordingly, the need remains for simplified systems with improved performance and reduced costs.

SUMMARY OF THE INVENTION

The design approach of the present invention treats all lens elements on each side of the intermediate image as part of one whole system. The cost and performance of lenses is improved by allowing large field curvature, focus, astigmatism, distortion and color separation at an intermediate image, then correcting those aberrations at the final image. Likewise, thermal correction is also performed on an intermediate image lens system as a single system. The position of the system stop is selected to improve vignetting and performance but the iris may be located at a different position.

Varifocal and zoom lenses with intermediate images are also improved using these methods and some new varifocal and zoom configurations are disclosed.

The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention that proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGs. IA- IF are schematic views of examplary lens systems implemented according to teachings of the present invention with the real intermediate image moving along an optical axis within or through moving lens groups with respect to a final image plane.

FIGs. 2a-2c illustrates a compound zoom lens shown in three focus positions with an intermediate image effected within a moving lens group per a first embodiment of the invention.

FIG. 3a-3c illustrates a compound varifocal lens in three focus positions with an intermediate image effected within a moving lens group per an alternate embodiment of the invention.

FIGs. 4a-4c illustrates a compound varifocal lens in three focus positions with an intermediate image effected within a moving lens group per yet another embodiment of the invention.

FIGs. 5a-5c illustrates a compound zoon lens in three focus positions with an intermediate image moving from one side of a lens element to another per still another embodiment of the invention. FIGs. 6a-6c illustrate a magnified portion of the intermediate lens group of FIGs. 5A-5C with ray crossings showing the curved plane of the intermediate image.

FIGs. 7-9 are plots showing examplary changes in focus, distortion, lateral color, and MTF between intermediate images and final images in the lens systems implemented according to the present invention.

DETAILED DESCRIPTION

FIGs. IA through IF illustrate various configurations of intermediate image lens systems in schematic form. Light from object 10 passes generally along optical axis 12 through a series of lens elements — e.g. front lens group 14 and rear/moving lens group 16 — where the light is focused onto an image plane at the back of the lens system to form a final image 18.

The specific lens elements selected for each lens group is based on the performance required, such as zoom, focal length, field of view, and f number. The lens elements selected further result in the formation of an intermediate real image 20 between the object 10 and

final image 18. The intermediate real image is a flipped version of the object 10 and, as will be appreciated from a description commencing below, is affected with optical aberrations including field curvature, focus, astigmatism, distortion, and color separation of potentially substantially greater magnitude that the corresponding optical aberrations of the final image 18.

FIG. IA illustrates an embodiment of the invention where the intermediate image is contained within a (rear) moving lens group 16, the front lens group 14 being stationary (optionally movable). The horizontal arrows with arrowheads on both ends in the upper portions of the figures indicate that each of the moving lens groups (e.g. lens group 16) are movable in both axial directions but in a monotonic manner (i.e. in only one direction when progressing from one extreme to the other of adjustments).

FIG. IB illustrates an embodiment of the invention where the intermediate image 20 is contained with a (front) moving lens group 22, the rear lens group 24 being stationary (or optionally movable). FIG. 1C illustrates an embodiment of the invention where the intermediate image 20 is contained within an intermediate moving lens group 26. The lens system of FIG. 1C further includes a front lens group 28, including at least two lens elements, located between the object 10 and the intermediate real image 20, and a rear lens group 30, including at least two lens elements, located between the intermediate real image 20 and the final image 18. The rear lens group 30 is separately moveable along the optical axis 12 from the intermediate lens group 26 to change the magnification of the final image 18.

FIG. ID illustrates an embodiment of the invention where the intermediate image 20 is contained within an intermediate moving lens group 26 just as in FIG. 1C, except that the front lens group 32 is separately moveable along the optical axis 12 to change the magnification of the final image 18.

FIG. IE illustrates an embodiment of the invention where the intermediate image is captured within a first moveable lens group 34. A second movable lens group 36 within the first moveable lens group 34, and located rearward of the intermediate real image 20 along the optical axis 12, is additionally moveable from the first moveable lens group 34. This design also contemplates third and fourth movable lens groups (not shown), separate from each other and the second movable lens group 36, that are within and additionally movable from the first movable lens group 34.

FIG. IF illustrates an embodiment of the invention where moving lens group 38 moves through the intermediate real image 18 as the magnification of the final image 18 is changed.

Some existing zoom lens designs describe their lenses as containing a front zoom section with sub-groups, an intermediate image, and a rear zoom section with sub-groups. The whole system designed lenses described here do not split the lens system into a front zoom section and a rear zoom section. Some lens systems do not have sub-groups in either the front or rear of the intermediate image. Some have a single sub-group that exists on both sides of the intermediate image and some have a group that moves through the intermediate image while zooming. Also current zoom lenses do not describe varifocal configurations.

The new design type treats all lens elements on each side of the intermediate image as part of one whole system. The cost and performance of intermediate image lens systems is improved by allowing large field curvature, focus, astigmatism, distortion, and color separation at an intermediate image and then correcting those aberrations at the final image. Likewise, thermal correction is also performed on an intermediate image lens system as a single system. The position of the system stop is selected to improve vignetting and performance but the iris may be located at a different position.

Varifocal and zoom lenses with intermediate images are also improved using these methods. New varifocal and zoom lens configurations have been invented that utilize the whole system design methods described above. These configurations differ from current zoom lens designs with intermediate images by the use of the whole system design methods with the emphasis on the final image and allowing the intermediate image to have poor performance.

The intermediate real image 18 is allowed to move as the magnification of the final image is changed by moving a lens group or groups. The intermediate real image 18 is fully contained within a moving group during some (e.g. lens group 38 in FIG. IF) or all (e.g. lens group 16 in FIG. IA) of the magnification changes. The intermediate real image can also move through one or more lens elements within the groups.

In a general embodiment, a lens system is comprised of moving and stationary groups of lenses and an intermediate real image between the object and the final image.

FIG. 2a-2c illustrate a lens system 40 of similar general structure to that shown schematically in FIG. 1C in three zoom positions. Lens system 40 illustrates a 2Ox zoom lens with performance as shown in Table 1, with the optical prescription of each of the lens elements shown in Table 2.

Stationary front lens group 28 includes a multi-lens structure with three singlets 42, 44, and 46, and a doublet formed of lenses 48 and 50. Front lens group 28 has a negative power. Intermediate moving group 26 is formed of two singlet lenses 52, 54 and captures the intermediate real image 20 between them. The intermediate moving group 26 is also referred to as a field group. It will be appreciated that, because of the whole system design, convergence of field plane points is not a flat plane but rather a curved surface, and generally aspherical in shape due to an allowance of aberrations at the intermediate real image so long as such aberrations can be corrected upon reaching the image plane at the final image 18. A rear lens group 30 moves together and includes a doublet, formed of lenses 56 and 58, and four singlets 60, 62, 64, and 66. Rear lens group 30 is typically a positive power moving group.

While only the lens elements are physically shown in FIGs. 2-6, it is to be understood that conventional mechanical devices and mechanisms are provided for supporting the lens elements and for causing axial movement of the moveable groups in a conventional lens housing or barrel .

Table 1 Lens Performance (FIGs. 2a-2c)

The lens construction and fabrication data for lenses disclosed herein, particularly for lens systems 40, 70, 80, and 90 are set forth in Tables 2, 4, 6, and 8 respectively. The data within the tables is expressed from data extracted from CODE V® optical design software that is commercially available from Optical Research Associates of Pasadena, Calif., U.S.A., which was also used for producing the optical diagrams of FIGs. 2a-2c, 3a-3c, 4a-4c, and 5a- 5d and performance plots FIGs. 7a, 7b, 8a, 8b, 9a, and 9b. Throughout this specification,

including the Tables, all lens radius (RDY) and thickness (THI) measurements are in millimeters (mm). The first column denotes the lens surface from that closest to the object (' 1 ') to the final image ('31 ') — left to right in the figures. The lens radius denotes the optical surface radius of curvature for each surface, with a minus sign (-) meaning the center of the radius of curvature is to the left of the surface, as viewed in FIG. 2a, and "Infinity" meaning an optically flat surface. The term "ASP" denotes and aspheric surface with the characteristics K, IC, CUF, and A-D understood by those in the optical arts. The thickness (THI) criteria denotes the axial distance between that surface and the next surface. The RMD characteristic expresses that the lens elements used are refractive (transmissive) and not reflective (e.g. mirrors). The GLA characteristic in the Tables expresses the type of glass used for the lens element.

Table 2

OpticalPrescription (FIGs. 2a-2c)

RDY THI RMD GLA

> OBJ INFINITY 1400.000000

1 18.88432 2.000000 SLAH66_ OHARA

2 7.75455 2.823385

3 25.99426 4.998361 LLAH81_ _0HARA

ASP:

K : -2.459394

IC : YES CUF: 0.000000

A : -.409744E-04 B : -.344073E-05

C : 0.101465E-06 D : -.172313E-08

4 -33.97649 1.695081

ASP:

K : 2.883487

IC : YES CUF: 0.000000

A : -.125169E-03 B : -.280912E-05

C : 0.543996E-07 D : -.969927E-09

5 -9.66063 3.866946 SLAL8_0HARA

6 -12.02896 0.500000

7 103.17049 1.000000 STIM35_ OHARA p 15.16159 4.634896 SFPL5l] _0HARA

9 -15.16159 1.000000

10 -10.63257 5.932709 SLAL13_ JDHARA

ASP:

K : -17.236145

IC : YES CUF: 0.000000

A : -.299517E-03 B : 0.113039E-04

C : -.264639E-06 D : 0.256522E-08

11 -5.49687 4.535214

ASP:

K : -1.033420

IC : YES CUF: 0.000000

A : 0.401631E-04 B : O.OOOOOOE+00

C : O.OOOOOOE+00 D : O.OOOOOOE+00

12 19.92247 5.499864

13 18.72772 4.500673

14 26.13597 3.131550 STIH6_OHARA

15 -597.96649 44.595211

16 -28.13051 2.000000 SFPL51_OHARA

17 -5.29715 1.373425 SLAH60_OHARA

18 -8.60980 0.500000

19 4.70204 2.327373 SBSMl6 OHARA

20 19.98879 1.000000

STO INFINITY 0.500000

22 3.78159 1.000000 SLAH66_OHARA

23 3.73681 1.121580

24 -4.02833 1.000000 STIH53_OHARA

25 3.99103 1.273248

26 7.46834 2.218536 LLAH81 OHARA

ASP:

K : -7.901658

IC : YES CUF: 0.000000

A : 0.199221E-02 B : O.OOOOOOE+00

C : O.OOOOOOE+00 D : O.OOOOOOE+00

27 -8.47590 2.271948

ASP:

K : -3.629684

IC : YES CUF: 0.000000

A : 0.862980E-03 B : 0.626744E-05

C : 0.120331E-04 D : -.796905E-06

28 INFINITY 0.700000 SBSL7_OHARA

29 INFINITY 1.000000

30 INFINITY 0.500000 SBSL7 OHARA

31 INFINITY 0.500000

IMG INFINITY 0.000000

FIG. 3a-3c illustrate a lens system 70 of similar general structure to that shown schematically in FIG. IE in three zoom positions. Lens system 70 illustrates a 1.5x varifocal lens with performance as shown in Table 3, with the optical prescription of each of the lens elements shown in Table 4. Lens system 70 comprises a moving group of lenses and an intermediate real image 20 between the object and the final image 18. The first moving lens group 34 contains the intermediate real image 20. A final moving lens group 36, rearward of the first group 34, is also additionally movable to the first group as the entire lens system can be moved as one unit. A fold prism 72 is located in the first moving group 34 to allow the optical path to be folded.

The varifocal lenses can be focused by moving the entire lens or they can be configured to lock the final image in place and allow the front group 34 to move, thereby increasing the moving group count by one. If the front group is moved with the same mechanism used to move the other groups, then these lenses become zoom lenses.

Table 3 Lens Performance (FIGs. Sa-Zc)

Table 4 Optical Prescription (FIGs.3a-3c)

RDY THI RMD GLA

> OBJ INFINITY 600.000000

1 -260.49084 2.000000 LLALl 3 OHARA

ASP:

K : 0.000000

IC : YES CUF: 0.000000

A : 0 .250324E-02 B : -.784408E-04

C : 0 .241381E-05 D : -.345479E-07

2 10.34057 1.663760

ASP:

K : 5.899479

IC : YES CUF: 0.000000

A : 0 .318065E-02 B : -.111097E-03

C : 0 .120676E-04 D : -.119459E-05

3 INFINITY 9.000000 STIH53_OHARA

4 INFINITY 0.672493

5 17.21594 1.040373 STIH4_OHARA

6 7.59611 4.624505 SFPL51_OHARA

7 -7.59611 0.515293

8 19.56185 2.477144 SLAL8_OHARA

9 -228.72490 0.504400

10 13.44299 2.827054 LLAH53 OHARA

ASP:

K : -0.351285

IC : YES CUF: 0.000000

A : - .159144E-03 B : 0.183781E-05

C : - .242340E-07 D : 0.339042E-09

11 INFINITY 5.402070

12 INFINITY 0.000000 100000.990000

13 INFINITY 5.270590

14 -13.41927 1.500000 STIM22_OHARA

15 12.32409 3.118736

16 -44.69275 4.075686 SLAH6OJDHARA

17 -9.89463 20.461145

18 8.51241 3.022636 SLAL8 OHARA

19. -100.96761 0.500000

20 : 9.66878 2.505078 SFPL51_OHARA

21 : -9.66878 1.045694 STIH53 OHARA

22 : 25.26628 1.000000

STO ; INFINITY 1.000000

24 : 3.74559 1.369335 STIH53_OHARA

25 : 2.46680 1.656541

26 : -2.18297 1.000000 STIH53_OHARA

27 -3.24518 0.500000

28 : -2794.02915 2.247511 LLAH53 OHARA

ASP:

K : 0.000000

IC : YES CUF: 0.000000

A : 0 .905661E-03 B : -.196452E-04

C : 0 .469950E-05 D : -.243452E-06

29 . -7.40373 1.300649

ASP:

K : -1.225778

IC : YES CUF: 0.000000

A : 0 .OOOOOOE+00 B : 0. OOOOOOE+00

C : 0 .OOOOOOE+00 D : 0. OOOOOOE+00

30 : INFINITY 0.700000 SBSL7_OHARA

31 INFINITY 1.000000

32 : INFINITY 0.500000 SBSL7 OHARA

33 : INFINITY 0.500000

IMG _: INFINITY 0.000000

FIGs.4a-4c illustrate a lens system 80 of similar general structure to that shown schematically in FIG. ID, but with an additional moving lens group 82, in three zoom positions. Lens system 80 illustrates a 5x varifocal lens with four moving groups, including front moving group 32, intermediate moving group 26, a second intermediate moving group 82, and a rear moving group 30. The intermediate real image 20 exhibits a distorted structure owing to the whole system design concept of the present invention where optical aberrations are allowed in the intermediate image. Accordingly, at least part of the intermediate real image 20 is within moving group 26, comprising a singlet lens 84, during zooming.

Performance of the 5x zoom lens is shown in Table 5, with the optical prescription of each of the lens elements shown in Table 6.

Table 5 Lens Performance (FIGs.4a-4c)

Table 6 Optical Prescription (FIGs.4a-4c)

RDY THI RMD GLA

> OBJ INFINITY 1400.000000

1 14.07003 3.000000 STIH53_OHARA

2 9.20552 1.407311

3 139.63133 2.000000 LLALl 3 OHARA

ASP:

K : 0.000000

IC : YES CUF: 0.000000

A : 0 .180871E-02 B : -.379204E-04

C : 0 .978093E-06 D : -.883183E-08

4 10.59640 2.423059

ASP:

K : 4.095093

IC : YES CUF: 0.000000

A : 0 .223689E-02 B : -.271386E-04

C : 0 .179148E-05 D : -.178184E-06

5 346.75882 5.000000 SLAH60_OHARA

6 15.30348 0.500000

7 9.55962 4.000000 STIH4_OHARA

8 6.82758 4.689415 SFPL51_OHARA

9 -8.90922 0.807245

10 42.01426 3.651561 SLAL8_0HARA

11 -20.86589 8.952950

12 16.55511 5.000000 LLAH53 OHARA

ASP:

K : -0.099465

IC : YES CUF: 0.000000

A : - .119734E-03 B : 0.225633E-05

C : - .303880E-07 D : 0.153960E-09

13 INFINITY 15.826803

14 -14.35737 1.500000 STIM22_OHARA

15 17.38403 4.222659

16 -59.06957 4.859917 SLAH60_OHARA

17 -13.15643 41.909638

18 13.80672 5.000000 SLAL8_0HARA

19 -46.32335 0.825960

20 13.09140 3.024270 SFPL51_OHARA

21 -13.09140 1.804414 STIH53_OHARA

22 28.69652 1.000000

STO INFINITY 1.000000

24 4.18707 1.729970 STIH53 OHARA

25 2.77530 1.938253

JO

26. -4.86096 1.422520 STIH53JDHARA

27 : -6.12738 0.500011

28 ; 60.78152 1.906038 LLAH53 OHARA

ASP:

K : 0.000000

IC : YES CUF: 0.000000

A :-, .119697E-03 B :-.827274E-05

C :0. .614529E-05 D :-.344937E-06

29 -12.75024 1.893191

ASP:

K : 5.958613

IC : YES CUF: 0.000000

A :0. .OOOOOOE+00 B :0. OOOOOOE+00

C :0. .OOOOOOE+00 D :0. OOOOOOE+00

30 . INFINITY 0.700000 SBSL7_OHARA

31 : INFINITY 1.000000

32 ; INFINITY 0.500000 SBSL7 OHARA

33 : INFINITY 0.500000

IMG . INFINITY 0.000000

FIGs.5a-5c illustrate a lens system 90 of similar general structure to that shown schematically in FIG.1C, but where the intermediate real image 20 moves through a lens element 92 within the intermediate moving lens group 26 as the magnification of the final image 18 is changed. The system 90 is designed as a 15x zoom lens. Lens system 90 includes a stationary front lens group 28, an intermediate moving lens group 26 (including lens element 92 and other singlet lenses 94, 96), and a moving rear lens group 30.

Performance of the 15x zoom lens is shown in Table 7, with the optical prescription of each of the lens elements shown in Table 8.

Table 7 Lens Performance (FIGs.2a-2c)

U

Table 8 Optical Prescription (FIGs. 5a-5c)

RDY THI RMD GLA

OBJ INFINITY 1400.000000

]_ 16.87758 2.000000 SLAH60_OHARA

2 7.16821 2.910261

30 3 23.69598 2.971174 STIHl 0_OHARA

4 -23.91718 1.625796

5 -9.27996 3.265211 STIH6_OHARA

6 -9.91002 2.379631

7 -36.73544 3.000000 STIM35_OHARA

I S 8 13.58046 3.746610 SFPL51_OHARA

9 -10.50476 0.500000

10 -41.26535 5.672691 LLAH53 OHARA

ASP:

K : -130.643869

20 IC : YES CUF: 0.000000

A : -.352862E-04 B : 0.354094E-05

C : -.542721E-07 D : 0.334914E-09

11 -8.12073 5.789277

25 ASP:

K : -0.833502

IC : YES CUF: 0.000000

A : 0.181968E-03 B : O.OOOOOOE+00

C : O.OOOOOOE+00 D : O.OOOOOOE+00

30

12 -55.16899 2.000000 SBSL7_OHARA

13 56.48198 5.222600

14 -44.84376 6.000000 SLAH6 OJDHARA

15 -17.83269 49.474826

35 16 10.94757 5.160549 SFPL51_OHARA

17 -6.76064 1.000000 SLAH60_OHARA

18 -32.82385 0.500000

19 7.65325 2.216490 SLAH66_OHARA

20 -35.25759 1.000000

40 STO INFINITY 0.500000

22 3.38080 1.000000 STIH6_OHARA

23 2.51532 1.421441

24 -3.63901 1.003159 STIH53_OHARA

25 6.32264 0.570596

45 26 9.73140 2.412720 LLAH53 OHARA

ASP:

K : -11.526595

IC : YES CUF: 0.000000

A : 0.191930E-03 B : O.OOOOOOE+00

50 C : O.OOOOOOE+00 D : O.OOOOOOE+00

27 -5.52474 1.865620

ASP:

K : 0.010552

55 IC : YES CUF: 0.000000

A : 0.183875E-03 B : O.OOOOOOE+00

C : O.OOOOOOE+00 D : O.OOOOOOE+00

28 INFINITY 0.700000 SBSL7 OHARA

60 29 INFINITY 1.000000

30 : INFINITY 0 . 500000 SBSL7_OHARA

31 : INFINITY 0 . 500000

IMG : INFINITY 0 . 000000

Field curvature at intermediate image:

A close up view of the intermediate image is shown in FIGs. 6a-6c. In the figures, the intermediate image is shown by the convergence of rays for each field point. Notice that the plane of convergence for all field points is not a flat plane but a curved surface. The curved surface is generally aspherical in shape.

The field curvature at the intermediate image is also shown in the astigmatic field curves of FIGs. 7a. The field curvature of the final image is show in FIG. 7b.

The intermediate field curvature is allowed to extend to any amount and is not constrained to a flat plane.

Distortion at intermediate image:

FIG. 7a shows the distortion at the intermediate image. The intermediate distortion is not constrained and is greater than the distortion at the final image, shown in FIG. 7b. In practice the intermediate distortion ranges from 5% barrel distortion to 100% or more barrel distortion while the final image distortion is less than the distortion of the intermediate image.

Lateral Color at intermediate image: The color separation, lateral color, at the intermediate image is allowed to be very large and undercorrected compared to the final image. FIG. 8a shows twenty-five times more lateral color at the intermediate image than FIG. 8b at the final image. In practice, the lateral color of the intermediate image can be greater than five times that of the final image and sometimes even greater than 100 times. By allowing the intermediate image to have poor color correction the designer can use fewer color correcting lens elements in the system thereby reduce lens element count and cost. Color correcting lens elements are positioned between the object and the intermediate image and between the intermediate image and the final image. These lens elements all work together to give acceptable color correction at the final image.

The ray aberration plots would also show the difference in the color correction of the intermediate image and the final image. For the intermediate image the colors are shown to be very separated.

Focus at intermediate image:

Focus at the intermediate image is not constrained and the MTF (modular transfer function, a measure of lens sharpness) performance is very low compared to the MTF of the final image. Inspection of FIGs. 6a-6c shows that the convergence of rays for each field point is poor. FIG. 9a shows that the MTF of the intermediate is about 1/10 ^th of the final MTF shown in FIG. 9b. Allowing the intermediate image to have very poor focus performance requires fewer lens elements between the object and the intermediate image and fewer elements between the intermediate image and the final image, thereby reducing cost and complexity. Since the system is designed as a complete system, no one or two groups are required to correct all aberrations and produce good imaging qualities. Instead, all lenses work together to achieve good performance in the final image.

Thermal correction at intermediate image:

Thermal correction is spread out across all lens elements on both sides of the intermediate image to result in a thermally corrected lens system. Thermally compensating glass types and lens shapes are chosen and positioned both before and after the intermediate image to thermally compensate the system from e.g. -20° C to +70° C. The lenses described herein show a poor thermal performance of the intermediate image at -20° C, but a good performance of the entire system at -20 C. Likewise, lenses described herein show a poor thermal performance of the intermediate image at +70° C, but a good performance of the entire system at +70° C.

Vignetting and stop position:

This lens type potentially has two stop positions; one between the object and the intermediate image, the front stop, and another between the intermediate image and the final image, the rear stop. Generally the best vignetting result occurs when the system optical stop is positioned at the rear stop. This position allows the front stop to be non-constrained. At the front stop the chief rays for each field point are not constrained to be in the same z position along the axis of the optical system. They are allowed to vary in z position and height above or below the optical axis. Also the volume of space used by each field at the

front stop is allowed to vary. The volume for each field at the front stop increases with increasing field. The increasing volume for the edge field points avoids the cosine squared vignetting that usually occurs at a defined stop with large incident angles such as is typically found in wide angle lenses. The edge field points do not have such large angles at the rear stop so they are less affected by the cosine squared rule. In this way vignetting performance is improved above what is normally available to rectilinear lenses and results in better overall transmission of light through the lens.

Iris position and stop position: Since there are two optical stop positions available, the lens iris, or mechanical stop, can be positioned at either or both stop positions. Since the front stop varies in position and size, it is often best to put the iris at the rear stop position. However, sometimes a mechanical constraint or some other consideration requires the iris to be positioned at the front stop. To maintain some of the vignetting advantages listed above, the optical stop is still positioned at the rear stop but the iris is positioned at the front stop. The designer can then control the edge rays at the front stop without controlling the chief rays at the front stop. This allows less vignetting of the edge field points, better transmission, and the flexibility to position the iris at the front stop. In one embodiment, the transmission of the edge field point is 57% due to the unconstrained front stop. When the front stop is constrained the transmission falls to 45%.

Other Lens Examples

1. A 3x varifocal lens that is foldable with a prism in the front group. There is one internal moving group between the object and the intermediate image.

2. A 7x varifocal lens with 2 moving groups. The first moving group contains lens elements that are on both the object and final image side of the intermediate image. The rear group is the second moving group.

3. A 15x varifocal lens with 2 moving groups. The first moving group contains lens elements that are on both the object and final image side of the intermediate image. One element in this group moves through the intermediate image. The rear group is the second moving group.

4. A 9x varifocal lens with 2 moving groups. Both moving groups are after the intermediate image.

5. A 5x varifocal lens with 2 moving groups. Both moving groups are after the intermediate image.

6. A 5x varifocal that is foldable with a prism in the front group. Both of the 2 moving groups are after the intermediate image. A folded version of the 5x varifocal of is possible through the prism. The lens system has no moving subgroups in the front group between the object and the intermediate real image. The magnification of the front group does not change as the lens system is zoomed. The rear group does change magnification.

In another version, the separate magnification of the front group and the separate magnification of the rear group do not change as the lens is zoomed. The total system magnification does change as the front and rear groups are moved.

Zoom lens systems can then be designed with a first lens group between the object and the intermediate real image with no moving subgroups wherein the focal length of the first lens group is not changed as the first lens group is moved. Furthermore, a second lens group is included between the intermediate real image and the final image with moving subgroups wherein the focal length of the second lens group is changed as the subgroups of the second lens group are moved. In an alternate embodiment, the two groups are flipped so that the first lens group has moving subgroups and the second has none. Furthermore, the moving subgroups can be exchanged for non- moving ones so that the focal length of each lens group is not changed as the group as a whole is moved.

7. A 5x zoom lens that is foldable with a front prism. It has two moving groups. In order; there is a fixed front group, a moving group, a fixed negative-positive field lens group, and a moving rear group. A folded configuration is possible using the same 5x zoom lens with the prism.

Zoom 5x

8. A front foldable zoom lens with two moving groups and the optical stop located in the front position. The diameter of the front stop must increase as the lens is zoomed from a wide angle to a lesser angle.

9. A 15x zoom lens with 3 moving groups: a fixed front group, a moving group, a moving field lens group and a moving rear group.

10. A 10x zoom lens with 2 moving groups: a fixed front group, a moving field lens group and a moving rear group. The intermediate image is contained within the moving field lens group.

IS

Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications and variation coming within the spirit and scope of the following claims.