Does the lens focal length change on different size film formats?

I've read that putting a Pentax 6x7 lens on a 35mm body doesn't change the lens focal length. But if I remember correctly from my digital days, if you have a DSLR w/ a crop sensor it does change the apparent focal length. Isn't a DSLR sensor the same as a film plane, or are they factoring this sort of thing in when they design the lenses for DSLRs?

You mix up two terms: "focal length" and "apparent focal length".

-) The focal length is inherent part of the optical design of the lens.
-) What you call apparent focal length is the focal length that would produce same angle of view on a reference image format. The kind of image capture (sensor, film, groundglass) does not matter.

The focal length stays unchanged when you switch formats but keep the lens unchanged.
The field of view changes when you switch formats but keep the lens unchanged.
When you see someone talking about a lens providing a "35mm equivalent" result, they are referring to a lens that gives one a field of view that is the same as the "35mm equivalent" lens on 35mm film.
When the field of view is the same, the camera position will likely end up being the same, and therefore the perspective will be the same.
The depth of field will differ.

If you mount the 90mm f/2.8 lens from the 6x7 on a 35mm camera it would work the same as a 90mm f/2.8 on a 35mm camera. But the 90mm on the 6x7 is a normal lens and on the 35mm it's a telephoto.

cliveh said:

So if you switch from 35mm to 10" X 8" with a 50mm lens the focal length stays unchanged. I think not.

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No change in focal length. And for the same subject distance, you would need to focus by having the film the same distance from the nodal point of the lens. That would be difficult - even a bag bellows with a recessed lens board would probably not let you have the film that close.
And as that lens is designed to cover a 35mm frame, it would only fill a small central part of the 8"x10" film with image.
If somehow you had a 50mm lens designed to fill the 8"x10" film - an extreme wide angle lens - it would be huge and difficult to mount on your 35mm camera, and the film in the 35mm camera would only use a very small centre portion of the image formed by the lens.
A more reasonable thought experiment would be to take a 90mm lens designed for 35mm film and mount it on the 8"x10" camera. Again, you would need to focus by having the film the same distance from the nodal point of the lens, although you could probably achieve that using a bag bellows and/or a recessed lens board.
And as that lens is also designed to cover a 35mm frame, it would also only fill a small central part of the 8"x10" film with image.

I remember experiments in my high school physics class 75 years ago in which we determined focal length of various lenses. The size of the paper upon which images were projected didn’t matter.

The reason that they are called "crop sensor" cameras is that they physically crop the image compared to what would be seen for a full 35mm frame with the same lens.

Various other limitations come into play, for example, the lenses made for crop sensor cameras often can't produce a large enough circle of light to illuminate a larger sensor/film area. But, the size of the image that it makes is the same as it is for any other lens with the same focal length, as noted above.

The optics don't change-- a 100mm lens with a 25mm aperture is going to be a 100mm f/4 lens, regardless of what size focal plane is being used.

If used with a 35mm full-frame camera, and an APS-C 1.6x crop factor camera, then the only difference should be that one produces a 36x24mm image, and the other produces a 22.3x14.9mm image. But the 22.3x14.9mm rectangle at the center of the image should be identical on both cameras.

Similarly, if that lens can cover 6x7 negatives, then if used with a 35mm camera, you should have a 36x24mm rectangle at the center of the 6x7 negative that exactly matches the negative captured on a 35mm camera.

wiltw said:

• Someone who is 6' tall and standing at 25.3' from focal plane of camera will be 24mm tall on film when a 100mm FL is used, no matter what the format!

• The only difference will be the amount of area surrounding the 6' tall person which is captured on film, with formats larger than 24mm vertical.

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Thanks Wilt, even I understood that. The field of coverage changes is what you're saying. It's trickier than I thought.

I started thinking about this after buying a LF meniscus lens from a member here some time ago, and while playing w/ it to see where the focus points were I realized that I needed to at least make a tube w/ a GG on the back. But then I thought, why not just put the lens on a tube and hold it up to the Nikon? It didn't work very well, so I need to mount it on a proper camera (4x5 probably) w/ a bellows and ck it out.