Zoom Lens

This invention relates to zoom lenses and more particularly relates to zoom lenses wherein the zooming range of equivalent focal lengths lies predominantly in what is referred to as a wide angle field of view.

The desirability of a compact small diameter, wide angle zoom lens particularly for use with single lens reflex cameras is well appreciated. The difficulties in designing such a lens, however, are extreme and multiple. If the lens is to cover approximately an 80.degree. field angle in its shortest focal length position, then the ratio of equivalent focal length to back focal length ratio must be about 0.625 or less. This condition determines the necessity of a retrofocus lens type.

Such a lens has been designed in a relatively simple form as shown in U.S. Pat. No. 3,143,590. The requirements for compactness and small diameter may be satisfied if the gaussian properties are properly selected and the entire zooming and compensating motions are achieved with basic negative and positive groups of the retrofocus lens groups. In the lens shown in the aforementioned patent, however, the aberrations are not well corrected. Specifically the distortion and coma vary excessively. One method of improving the aberration corrections is to employ multiple lens motions as shown in U.S. Pat. No. 3,771,853. This lens has achieved improved correction, partially due to the variation of spacings of lens elements during zooming.

It is not necessary to add additional motions to the two basic component motions first described. If the gaussian properties are properly selected a field angle of 40.degree. to 80.degree. may be achieved with as few as nine elements, if such elements are constructed and arranged in the proper manner.

Such improved properties are achieved in the present invention through the provision of a lens having a first negative grouping and a second positive grouping. During zooming,, from the lower focal length to the upper focal length, the rear positive grouping moves forward while the front negative grouping moves rearwardly maintaining the position of the image plane.

The first grouping in one form thereof comprises from the object to image end a first positive group in the form of a convex object side meniscus, a second group in the form of a plurality of negative menisci and a third positive group in the form of a meniscus having a convex object side surface. The second grouping comprises a first positive group including a cemented doublet and a second positive group of fairly conventional construction.

The negative grouping presents to the positive grouping a converging beam for field rays and a diverging beam for axial rays. For this reason, the aberration correction at the two ends of the focal length range tend to conflict. In the negative group, it has been determined that introducing a final positive group slightly air-spaced from the preceding negative elements provides correction for astigmatism and distortion, particularly at the lower focal length without introducing large amounts of spherical aberration, and further that coma may then partially be corrected in the second grouping. Additionally, it has been determined that using a positive element in the first position of the negative grouping further improves the distortion and astigmatism correction without adding uncorrectable aperture dependent aberrations. This is particularly important in a lens of the type where the aperture defining mechanism moves during zooming and the aperture thus requires modification as a function of focal length in order to maintain constant relative aperture. For example, the diameter of the entrance pupil when at the longer focal length range may have a maximum relative aperture of f/3.5. If the same diameter were maintained at the shorter focal length range, the relative aperture would be f/1.6. The lens powers of the first grouping are selected to permit simultaneous correction of distortion and lateral color while having a minimal contribution to spherical aberration. With this accomplished, the second grouping can then be of simplified construction, also being corrected for distortion.

An object of this invention is to provide a new and improved zoom lens.

Another object of this invention is to provide a new and improved zoom lens where the equivalent focal length range of the lens is predominantly less than the diagonal of the image frame.

A further object of this invention is to provide a new and improved wide angle zoom lens of the retrofocus type in which aberrations are reduced to an insignificant and acceptable level.

The features of the invention which are believed to be novel are particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to its operation and organization together with further objects and advantages thereof may best be appreciated by reference to the following detailed description taken in conjunction with the drawings wherein:

FIG. 1 is a diagram of a lens embodying the invention;

FIG. 2 is a chart showing the lens group movements as a function of equivalent focal length of the lens of FIG. 1; and

FIGS. 3-5 are diagrams of additional lenses embodying the invention.

In the following description each element is identified by L followed by a numeral. The two major groupings are identified by GP1 and GP2, and the surfaces are identified by R followed by a numeral.

The lens in all forms comprises a first grouping GP1 of negative power and a second grouping GP2 of positive power. Grouping GP2 moves forward to increase the equivalent focal length of the lens while grouping GP1 simultaneously moves rearwardly with motions as shown in FIG. 2. Such motions may be achieved through the lens mechanism disclosed in co-pending application Ser. No. 750,828 filed Dec. 15, 1976, now 4,141,636.

Grouping GP1 of FIGS. 1, 3 and 4 comprises four elements L1-L4 arranged as a first positive group element L1, a second negative group, elements L2 and L3 and a rear positive group, element L4. Elements L1-L4 move in fixed relation to each other.

Grouping GP1 of FIG. 5 comprises a first positive group element L1, a second negative group, elements L2, L3 and L3a, and a rear positive group L4. Elements L1-L4 move in fixed relation to each other.

Grouping GP2, as shown in FIG. 1, comprises elements L5-L9. Elements L5 and L6 comprise a relatively thick doublet having both forward and rear surfaces convex, and a rear positive group comprised of elements L7, L8 and L9 generally in the form of a Cooke triplet. Tables I and II set forth prescriptions of lenses as shown in FIG. 1.

In all examples grouping GP2 comprises a front positive group of relatively large thickness between front and rear convex surfaces along the optical axis, L5, L6 (FIGS. 1, 4 and 5; and L6, L7 and L8 (FIG. 3); and includes a bi-convex doublet. In the forms shown where the maximum relative aperture is approximately f/3.5 or greater the rear grouping conveniently takes the general form of a Cooke triplet as shown in FIGS. 1 and 3, and 5, or a modification with an additional negative element as shown in FIG. 4. If the lens is designed for a greater relative aperture, then other forms of the rear group of the second grouping may be utilized.

Two forms of the lens of FIG. 4 are set forth in Tables IV and V. In the lens of Table VI the aperture stop is placed closely before surface R9, while in the lens of Table V, the aperture stop is located closely behind surface R11. In the lens of Tables IV and V, the rear group of grouping GP2 is modified from that of FIGS. 1 and 3 by addition of the meniscus L9. FIG. 5 and Table VI exemplify a lens where the overall negative power of the front grouping GP1 is divided into three negative elements L2-L3a.

The prescriptions of lens of FIGS. 1 and 3-5 are set forth in the following Tables I-VI.

In the following tables, the lens elements and surface radii are designated with reference to the Figures. The axial distance between the radial surfaces of the elements is measured along the optical axis. N.sub.d is the index of refraction and V.sub.d is the Abbe number as derived from the glass identification number.

A lens as shown in FIG. 1 scaled to an equivalent focal length of 24.5 to 47.00 mm for a 24.times.36 mm image frame and having a relative aperture of f/3.65 is set forth in Table I.

The back focal length varies from 40.0 mm to 60.1 mm as the equivalent focal length of the lens varies from the lower value to the upper value. The axial spacing between groupings GP1 and GP2 decreases to 0.63 mm at the upper equivalent focal length.

A lens as shown in FIG. 1 scaled to an equivalent focal length of 24.5 to 47.0 mm for a 24.times.36 mm image frame, and having a relative aperture of f/3.65 is set forth in Table II.

The back focal length varies from 40.0 mm to 59.8 mm as the equivalent focal length of the lens varies from the lower value to the upper value. The axial spacing between groupings GP1 and GP2 decreases to 0.01 at upper equivalent focal length.

A lens as shown in FIG. 3 scaled to an equivalent focal length of 24.5 to 47.0 mm for an image frame of 24.times.36 mm and having a relative aperture of f/3.8 as set forth in Table III.

The back focal length varies from 40.0 mm to 59.9 mm as the equivalent focal length of the lens varies from the lower value to the upper value. The axial spacing between groupings GP1 and GP2 decreases to 0.50 mm at the upper equivalent focal length.

A lens as shown in FIG. 4 scaled to an equivalent focal length of 24.5 to 47.5 mm for an image frame of 24.times.36 mm, and having a relative aperture of f/3.8 is set forth in Table IV.

The back focal length varies from 39.0 mm to 58.3 mm as the equivalent focal length of the lens varies from the lower value to the upper value. The axial spacing between groupings GP1 and GP2 decreases to 0.1 mm at the upper equivalent focal length.

A lens as shown in FIG. 4 scaled to an equivalent focal length of 24.5 to 47.5 mm for an image frame of 24.times.36 mm, and having a relative aperture of f/3.8 is set forth in Table V.

The back focal length varies from 39.0 mm to 58.4 mm as the equivalent focal length of the lens varies from the lower value to the upper value. The axial spacing between groupings GP1 and GP2 decreases to 2.29 mm at the upper equivalent focal length.

A lens as shown in FIG. 5 scaled to an equivalent focal length of 24.5 to 47.0 mm for an image frame of 24.times.36 mm, and having a relative aperture of f/3.65 is set forth in Table VI.

The back focal length varies from 40.1 mm to 59.1 mm as the equivalent focal length of the lens varies from the lower value to the upper value. The axial spacing between groupings GP1 and GP2 decreases to 1.0 mm at the upper equivalent focal length.

As previously stated, it is the general form of the first grouping which is most important and particularly the positive groups thereof. The final positive group, air spaced from the preceding negative group, permits the provision of correction for astigmatism and distortion without introducing large amounts of spherical aberration at the wide angle limit of the zoom range and permits coma to be corrected in the second grouping.

The provision of a first positive group, element L1, in the front negative grouping improves the distortion and astigmatism correction without adding uncorrectable aperture-dependent aberrations.

In the simplest form the negative group of grouping GP1 comprises two negative elements. The negative power of the first grouping may be further subdivided into three elements as shown in FIG. 5; resulting in further improved distortion correction without introducing aperture-dependent operations which would be difficult to correct.

The second grouping consists of two main groups, the first having convex object and image side surfaces and including a cemented doublet. This group images the virtual object presented by the first front negative grouping at a very great distance, essentially infinity. The second group forms a real image at the film plane. The placing of the aperture stop between the two groups of the second grouping, or in front of the first group permits a smaller diameter of the front negative grouping without excessive vignetting. The first group, which is relatively thick between subtending convex surfaces and includes a bi-convex doublet, contributes to the positive power of the lens with minimum contribution to field curvature due to its thickness.

The EFLS, in millimeters, of the elements and groupings of the various lens are set forth in the following table.

It will be noted that in all cases the second positive group of the first grouping has a convex object side surface and a thickness substantially one-sixth of the lower EFL of the lens.

The spacing between the last negative element of the first grouping and the last positive group varies from 0.10 to 0.25 of the lower EFL of the lens.

The first positive group of the first grouping has a thickness of 0.15 to 0.35 of the lower EFL of the lens.

When the foregoing ranges of EFL's and thicknesses are maintained the lens is well corrected for the aberrations discussed in the introductory portion of this specification.

The axial thickness of the first group of the rear grouping is between one-fourth and three-fourths of the lower EFL of the lens.

The EFL of the rear positive group of the first grouping is always greater than twice the lower EFL of the lens. The ratio of the EFL of the first positive group of the first grouping to the EFL of the rear positive grouping is between two and five, and the EFL of the first positive group of the first grouping is at least five times the lower EFL of the lens.

The foremost negative element in the front grouping has an EFL from one to two times the shorter EFL of the lens.

Various modifications may be made in the disclosed lenses. For example, the lower EFL may be increased by decreasing the spacing between groups GP1 and GP2. The lens of Table I may be changed to a 28 mm to 47 mm EFL range by decreasing the spacing between elements L4 and L5 to 18.5 mm. The rear positive element L4 of grouping GP1 may be made as a doublet to correct for aperture dependent aberrations if the lens is designed with a larger maximum relative aperture.

In all disclosed embodiments, focusing is accomplished by independent movement of grouping GP1.

It may thus be seen that the objects of the invention set forth as well as those made apparent from the foregoing description are efficiently attained. While preferred embodiments of the invention have been set forth for purposes of disclosure, modification to the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments of the invention and modifications to the disclosed embodiments which do not depart from the spirit and scope of the invention .