Are lens for film cameras designed for a curved image plane?

I am considering getting a D50, but want to pick lenses first and am

looking at the 50mm AF f1.8D, which, I believe, is compatible with the

D50. I thought that lenses for 35mm film cameras were designed for a

curved image plane to conform with the surface of the film. I assume

that a D50 image sensor is flat. The APS-sized image sensor is about

half the size of 35mm, which is a relatively flat part of the film

surface, but it is not flat. Does that mean that when using a lens

designed for a film camera on a DSLR that the lens is suboptimal and

how much difference does it make? Thanks for any help.

Owen

Isn't the only point of a film pressure plate to keep the film flat?

Shakil's right, the pressure plate keeps the film flat. Any Nikon lens from 1977 (I think) is compatible with the D50, as far as I know.

Except for minor variations, lenses are designed to have a flat field of focus at both the film and subject planes. The film, incidentally, is held as flat as possible, not curved as you suggest. The sharpest focus at the subject is also a plane, parallel to the film.

Seems like I have read that some of the little Minox cameras were designed with a curved film arrangement. But all 35mm and larger cameras are built with flat film holding system, not curved.

The Complan Minox cameras have a curved image plane. But they no longer make Complan

Minox cameras.

 

Your Nikon lenses were all designed for a flat image plane.

Just taking the question a little further: Is the focus plane designed for a flat object, or for objects equi-distant from the lens?

 

ie. A photograph of a wall with a wide angle lens, with the center of the image distance being 2m, may have a distance to the wall of 3m or more at the edges of the photo. Is this in focus, or only objects at 2m from the lens? Are there lenses which are designed differently on this aspect?

 

A wall may not be an interesting subject, but the question is relevant in a group photo: With a wide lens, is it better to position everyone in an arc, or in a flat image plane? From a perspective point of view, it would be better to have everyone in an arc with the center on the camera, so everyone is relatively the same size... but will they be perfectly in focus? (I am aware that DOF is large for wide angles, but for a small group photo with a standard prime lens at f1.8, it would matter...)

Christophe,<p>In looking over many lenses, I have found that they have curvature of field in the object space. That is, the optimum plane of focus for the subjects is in a section of a sphere rather than a flat plane. For example, the 35mm f/1.4, 50mm f/1.2, 58mm f/1.2 Noct, 50mm f/1.4 have some curvature of field. However, I must emphasize that this is a realtively small effect. I am testing at minimum focus distance, i.e. 18 inches or so. The curvature of field is visible as softening up of the corners when the camera is placed on a copy stand, photographing a flat document. Since other abberations may do this, I can verify the field of focus by using a pencil at the edges, and by bringing the pencil tip closer to the camera, the corner then comes into focus. Again, the distances involved are small - in the order of millimeters. Stopping down 2 stops gives enough depth of field and elimination of spherical abberation to sharpen up the corners significantly.<p>Other lenses, such as the 45 f/2.8 P, and 55-60mm Micro Nikkors are designed to give a flat object field. They can be used to copy a flat document. The corners and the center are sharp even wide open. They are also essentially distortion-free. The penalty is that they have some light fall-off wide open. So, you still end up closing down 2 stops for optimum image quality.<p>All Nikkor lenses expect the image plane (film or digital sensor) to be flat and at the correct plane indicated by the index mark on the camera (a circle with a line through it).<p>As to whether the curvature of field is a big issue with wide angles, the only way to know is to try it. With a big group shot, I'd suggest an arc arrangement of the subjects anyway. The people on the edges tend to "grow" if you keep everyone in a straight line. This distortion is your optical penalty for trying to compress more of the outside world onto the same size of film/sensor. If you arrange the subjects in an arc around your wide angle lens, the subject to film/sensor distance becomes more constant, and this wide angle "distortion" is minimized.

Christophe, most lenses are designed to have a flat field of focus for the subject. I am not aware of any Nikon lenses designed to have a curved subject plane (but in some speciallized cameras for astronomy). The subject should in a plane parallel to the film plane (Scheimflug effect notwithstanding), not equidistant from the lens.

 

There are issues of curvature of field, in that the correction of most lenses is not perfect. These defects are considered aberations, and are simply design compromises not intent.

film flatness can actually be a problem that affects sharpenss in 35mm film cameras, but mostly it happens with medium format cameras. It is one advantage that digital cameras have over film cameras: the pick up is 100% perfectly flat all the time.

In answer to the original question, the 50mm lens will work on the D50. Nearly all lenses designed for film Nikons can be used on the newest digital bodies (with some exceptions). The film lens offers a larger image circle than the digital-designed DX lenses, but are still usable. The DX lenses, however, when used in film bodies, throw the image circle on the center of the film, resulting in a circular photo with dark edges (like the circular fisheye lenses).

Thanks for the answers.

Owen

They generally figure the ideal lens would focus at objects on a plane, not objects equi-distant from the lens.

Edward Ingold is correct of course.

 

You can confirm this by experiment. If you pose a group of people in a straight line, parallel to the film plane, and take a photo focused on the center person with the lens wide open, you will see that the people at the ends are in focus. Repeat posing the group in a curve so that those at the ends are the same distance from the camera as the person in the middle. The image will show those at the ends out of focus unless you stop the lens down.

Depends on the lens, Richard. Many lenses exhibit some field curvature.

Of course, Andrew. Just as some posters exhibit OCD.

Guys, I see 2 answers to my question on wide angle focus plane: curved and flat, depending on the lens. I liked Robert's reply, it made sense, but then Edward and Richard claimed the opposite.

 

Are wide angle lens designed to shoot group photos and keep focus but distort the heads of the individuals, or designed to shoot group photos in an arc and keep focus and correct geometry for everyone? Are some lens different?

 

I often have 3 subjects in a close portrait at low DOF: would you place them on a flat plane, or equi-distant from the camera?

Well, no response, so I did a few test shots with a common scenario: 3 objects, 12-24 @18mm, and the results surprised me:

 

The lens is designed to eliminate the different distance effect between center and side subjects, so that the focus and perspective are best on a flat focus plane, not equi-distant from the lens. Below are 2 images, carefully calibrated either equi-distant from the camera, or on a flat plane. The equi-distant subjects are distorted at the sides, whereas the flat plane objects are geometrically correct. Focus difference is indistiguishable, f4 gives enough DOF at this focal length.

 

I conclude, for a group photo, keep everyone on the same flat plane.<div></div>