Field curvature effects minimized in tele/pan camera

Insertion of a planar element of plastic into the optical path between camera taking lens and the film, or rack and pinion rotation of the lens is a threaded lens mount, under the control of the device by which the user selects that particular pseudo or other image, adjusts the compromise setting of the lens to provide improved focus in the film area receiving the selected type of image.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to selectively improving overall image sharpness in a 
camera used in making pseudo format images such as pseudo telephoto format 
and pseudo panoramic format images. In such pseudo images, only a selected 
portion of the exposed film frame contains the image portion to be viewed, 
either directly or by printing concerned with cameras wherein the lens 
focus suffers from such large magnitude field curvature that the film 
plane is located at a compromise position in which some image sharpness at 
the center is sacrificed for better average overall sharpness. Such field 
curvature would ordinarily be encountered in a low cost lens. The 
invention provides means for adjusting lens focus in a simple, inexpensive 
manner, as the user selects the desired type of image, such that the 
compromise is adjusted to provide improved average sharpness in the film 
area which will receive the particular image type selected by the user. 
2. Description of the Prior Art 
I am not aware of any prior art teaching the concept of this invention. A 
discussion of pseudo photography can be found in U.S. Pat. No. 4,650,304, 
issued Mar. 17, 1987. As discussed in that patent, a pseudo telephoto 
print is one that is made from a central portion of an exposure having the 
same width-to-length ratio, e.g. 31/2:5, as that of the exposure. During 
the printing process, the negative is masked at its upper, lower, left and 
right marginal zones, leaving the central portion of the negative 
unobscured. Alternatively, the film gate in the enlarger may be 
correspondingly masked or adjusted. Then, an enlargement is made of the 
central portion of the negative to provide a print the same 
width-to-length ratio as that of the central portion. Thus, the print will 
have a telephoto or close-up format. 
A pseudo panoramic print is one that is made from a narrow portion of an 
exposure having a greater width-to-length ratio, e.g., 1:3, than that of 
the exposure. During the printing process, the negative is masked at its 
upper and/or lower marginal zones, leaving a narrow portion of the 
negative unobscured. Alternatively, the film gate in the enlarger may be 
correspondingly masked or adjusted. Then an enlargement is made of the 
narrow portion of the negative to provide a print with the same 
width-to-length ratio as that of the narrow portion. Thus, the print will 
have a panoramic or elongate format. 
SUMMARY OF THE INVENTION 
According to the invention, I improve camera of the type which takes pseudo 
images in different formats, such as pseudo telephoto format and pseudo 
panoramic format images. Low cost cameras of such type use inexpensive 
lenses which ordinarily suffer from field of such large magnitude, meaning 
that their focus is not planar, but is curved to the extent that the 
camera is designed with focus at a compromise position, sacrificing some 
sharpness at the center of the image to provide better average sharpness 
overall. 
If a normal (non-pseudo) image is selected, the center focus would be 
substantially behind the film plane and the edge focus would be 
substantially in front of the film plane. 
When only a central portion of the film frame is to be printed, as in a 
pseudo telephoto format image, a better compromise is attained by 
averaging the sharpness only over the area to be printed, namely, the 
central area of the film frame, leaving the edges to be much more out of 
focus than for a normal image, since the edge area will not be printed. 
This is accomplished by moving the central focus closer to the film plane. 
In the panoramic format mode, a bit less adjustment by movement of central 
focus would be made, as the width of the area to be printed would be 
narrow from top to bottom, and would be wider than in tele mode but 
narrower than in the normal mode. According to my invention, I provide on 
the camera, format selecting means controlled by the user, and means 
operated by the selecting means for placing the center focus of the lens 
at an optimum spacing from the film plane for the format selected, to 
optimize the focus over the film plane area which receives the image for 
that format. 
The foregoing explains the inventive concept, which can be carried out in 
various ways. In a preferred embodiment planar plastic means is inserted 
between the lens and the film, the thickness of which planar means 
determines the amount of focus adjustment. The planar material is 
optically clear and conveniently is a synthetic plastic material. The 
insertion is controlled by the device used by the operator to select the 
desired type of image. 
Another way of carrying out the invention is to mount the camera lens in a 
threaded mount and provide rack and pinion lens rotating means under the 
control of the device for selecting the type of image (mode selector).

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention is disclosed as being embodied in a low priced camera, in 
which minimum manufacturing cost is essential. In such cameras, a very 
inexpensive lens must be used. The manufacturing cost of a sophisticated 
lens with multiple elements to achieve flat field is unacceptable. 
Unfortunately, simple, inexpensive lenses suffer from large magnitude 
field curvature, that is, the sharpest focus lies in a curve, rather than 
a plane. With such lenses, the film plane and the lens would ordinarily be 
located in such positions with relation to each other that the curved 
field is in a compromise position relative to the film plane, the center 
of the curve (centers focus of the lens) being behind, and the edge focus 
of the lens being in front of, the film plane. In such position, some 
sharpness in the center is sacrificed for better average sharpness 
overall. 
Referring now to FIG. 1, there is indicated a camera lens 10 of a simple, 
inexpensive type. The aperture is indicated at 11, and the film plane is 
at 12. The "plane" of best focus is not a plane but is a curve indicated 
in exaggerated degree for clarity at 13. Dimensional arrow 14 indicates 
the diagonal of the film frame area to be printed (full area in this 
case). The spacing of the center of the curve (center focus of the lens 
with relation to the film plane is indicated at x. This is the largest 
spacing of any of the modes illustrated, being for the normal mode, that 
is, exposing a negative of which the entire frame will be printed. 
FIG. 2 shows the reduced film area diagonal of the image that is to be 
printed in the pseudo panoramic mode. In this mode, an elongated (from 
side-to-side) area of narrower dimension from top to bottom will be 
printed, giving the appearance of a panoramic photograph. Note that the 
location of the curve of best focus has been moved from the position shown 
in FIG. 1, so as to provide the best compromise focus only over the 
diagonal of the area to be printed, this being indicated at 15 and the new 
focus curve location being at 16. At the center of the film frame, the 
curve is closer to the film plane than in the normal mode, as indicated at 
Z. 
FIG. 3 shows the desired relocation of the focus curve 17a for the pseudo 
tele mode. In this case, only a central area of the film frame will be 
printed, as indicated by the shorter diagonal dimension indication 17. In 
this case the spacing Z has been reduced even more, since average 
sharpness is now concerned with a still smaller area to be printed. It 
will not matter that the image in the film areas outside the small area 
indicated by 17 will be severely out of focus, as these areas will not be 
printed, as was also the case for the areas outside the pan mode area. 
In a preferred embodiment, I achieve the desired positioning of the focus 
curve by inserting element 18 to improve the location of the focus curve. 
The element (which could consist of more than one element) is inserted 
between the lens and the film. It is a planar piece of optically clear 
material, conveniently of synthetic material. The element 18 is mounted in 
the camera for movement with a mode setting selector 19 (image type 
selecting means) by which the user selects the image type that he wants to 
have printed. In this case, the image type selector, which of course is 
located on the outer casing of the camera so as to be user accessible, 
cooperates with indicia 20 which indicate normal, panoramic and tele image 
types. Movement of the selector moves the element 18 in the space between 
the lens and the film and transversely of the optical axis of the lens. 
Element 18 has a thicker portion or area 21 and a thinner portion 22. It 
will be noted that the particular area, 21 or 22 positioned to affect 
light passing to the film, depends on the mode selected by the user. 
As shown in FIG. 4, for normal mode use, in which the entire film frame 
will be printed, the thicker area 21 is at operable position. In FIG. 5, 
the panoramic mode has been selected and the thinner area 22 of element 18 
is at operable position between the lens and the film. In FIG. 6, element 
18 has been withdrawn entirely to an inoperable position for a pseudo tele 
exposure. Thus in the example shown, the design provided the best average 
sharpness for the particular mode selected, the focus being adjusted by 
interposing the plastic element to a suitable extent or removing it. 
The plastic element 18 is dimensioned in accordance with the following 
equation 
EQU thickness=x-z(n/n-1) 
wherein n is the optical index of the plastic element, z is the desired 
spacing of the center focus of the lens from the film plane for the 
particular image selected, and x is the largest acceptable spacing of the 
central image focus from the film plane for the various formats for which 
the camera is to be used, this being the spacing in the normal mode for 
the embodiment disclosed. "Thickness" is measured along the lens axis, 
that is, perpendicular to the film plane. The lens axis is indicated by a 
horizontal line extending through the center of the lens in each Fig. of 
the drawings. 
In FIGS. 7 and 8 I have illustrated another mode of carrying out the 
invention. In this case the mode setting selector is indicated at 25, with 
the setting indicating indicia at 26. The camera wall is indicated at 27. 
Lens 28 is in a threaded mounting 29 such that rotation of the lens causes 
a change in focus. An annular portion 30 of the lens assembly has gear 
teeth about its periphery. These teeth engage a rack 31 carried by a link 
32 to selector 25. 
In operation, sliding movement of selector 25 during mode selection causes 
rack movement to rotate gear 30 and lens 28, causing focus adjustment. 
The invention has been described with reference to a preferred embodiment. 
However, it will be appreciated that variations and modifications can be 
effected within the ordinary skill in the art without departing from the 
scope of the invention.