Originally posted by Oog Compression or the appearance of "compression" or expansion has something to do with distance ratios. Take a head and shoulders shot - one with your favorite 85mm, and one with 24mm, on your favorite 35mm film. **Move close with the 24mm to match subject size.** The face and nose appears flattened with the 85mm. The nose looks longer and the hairline appears to recede with the 24mm.
Why?
For 85mm, the distance ratio of nose to camera:hairline to camera is close to 1:1.
For 24mm, the distance ratio of nose to camera:hairline to camera is considerably less than 1:1
We would normally refer to this apparent lack of compression when using the 24mm in this case as "subject distortion."
(and yes, I got the concept of distance/perspective 15 years ago....)
with regards to the topic of equivalence, the only factor that would remain equal is exposure. f1.2 would still be f1.2 and f2.8 would still be f2.8 regardless of whatever camera size sensor the lens is fitted at.
trying to equal the other factors would result to compromises.
equaling the FOV would still result to DOF difference. cropping the image from two different lenses could lead to less resolution from the shorter focal length lens and would still show DOF difference. compression would take place if focus distance is made a compromise.
equaling the DOF would mean difference in focal length, which means loss of FOV flexibility when shooting or shooting distance flexibility.
we try so hard on trying to equate two different systems. I'm not saying it is not achievable. it is achievable, to a certain extent, but as of the moment, with the present lenses that are available and the nature of the lenses themselves, there will always be compromises involved.
there is no real and perfect equivalent lens like the FA31 for the APS-C. no equivalent for the 50/1.2. nor the 85/1.4. all these would result to compromises like FOV (cropping), DOF difference and compression (focus distance involved). again the only thing that doesn't change is exposure.