Lens Definition Needed

You can focus on a fence rail 5 feet away with a macro lens wide open and the depth of field may only be 1 or 2 inches. Yet, the edge of the fence which is at the side of the frame may be about 5.7 inches from the lens and yet is in focus the same as the fence part that is in the center of the frame. What is this lens or feature called? (As a lens that shows no barrel or pincushion distortion is called a rectilinear lens.)

I am aware of "flat field lens" but I think there is a more scientific name.

 

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I am aware of "flat field lens" but I think there is a more scientific name.

I never thought much about flat field lenses until Kodak made the curved field lenses for

Carousel projectors. Since slides without glass are curved, it helps correct for that.

So I think flat field sounds right.

There are cameras where the film is intentionally not flat.

My first ever camera, that I didn't use very long, the Imperial Delta

for 127 film, has a curved (cylindrical shape) film guide. It is a simple

camera, maybe one element plastic lens, so I am not sure that they worked

so hard to get the shape right.

More recently, I got an Argus 3D camera, which puts two views onto a 35mm

full frame image, with a small overlap. Negatives will print on a single print

for two eyed viewing. The field is at least curved enough that the two halves

are in different planes, with a sprocket wheel in between. I believe that each

half has some curve, but I might have forgotten. The two halves go into mirrors

and lenses, to come out appropriately spaced for stereo viewing.

I believe some medium quality 35mm cameras also have curved film guides,

but haven't thought about it recently.

Seems to be described here:

Petzval field curvature - Wikipedia

Many enlarger lenses are made flat-field for projection onto the base board. This is one reason those are good for close-up macro work when mounted on a camera.

For the same reason, some macro lenses are made to minimize curvature of field. The inside-the-camera focus on 'normal' lenses has to be flat field too, which is why such lenses are reversed for macrophotography

I recall at least one camera which had a curved film path, a panoramic camera with a rotating lens. Another was the 48" Schmidt reflector at Mt. Palomar (now retired). Field curvature is 3D and symmetrical about the lens axis. You can only curve film on one axis without stretching, which is what the Schmidt did with a vacuum platen. Drum scanners and the Hasselblad Flextite used this to advantage. In both cases the film is stretched over a curved path and scanned on a single line relative to the axis of curvature, at a spacing which defines the resolution.

A rectilinear lens has, in theory, a flat field of sharpest focus in both the film plane and the subject. A photograph of a chess board held at right angles to the lens axis will be composed of equilateral squares on the film plane. If the field of view were a curve centered on the lens, a flat subject would appear curved away from the center. This would constitute a cylindrical (or spherical) projection (which is the first step in creating stitched panoramas).

That is why a fence rail is in focus in the center and ends, even though the ends are further away. It is also why curving a line of subjects to be equal distance from the lens is counterproductive.

Nothing is perfect, and a rectilinear lens is subject to linear distortion and field curvature. A fisheye lens is one in which barrel distortion is built-in to an extreme. Distance matters. Macro lenses are designed to have a flat field at close range (< 1:4), compared to normal lenses which are designed to work at 1:15 or more.

In photography, a rectilinear lens is a photographic lens that yields images where straight features, such as the walls of buildings, appear with straight lines, as opposed to being curved. In other words, it is a lens with little or no barrel or pincushion distortion

In flat-field, it is the PLANE of focus that is flat, as opposed to the more normal lenses that lose focus at the corners, for example.