Yes, this subject again. But I have a slightly different question and I was unable to find an answer here or anywhere else.
I understand a 50mm lens is a 50mm lens ALWAYS. I understand that "equivalent focal length" is a theoretical thing that is more about comparing field of view between different sensors. I understand that a 50mm DX on a cropped sensor and a 75mm FX on a FF have the same field of view. What I would like to know is: Do they have the same perspective? If you take a photo with a FF camera and 75mm DX lens and compare that image with another shot of same subject made by a cropped camera and a 50mm lens what will you see? Can they be superimposed without any deviation?
Another experiment: Two shots of same subject, one in a FF/50mm FX and another in a cropped camera/50mm DX. In this case, the image produced by the FF camera is large than the other. Fine. But if you superimpose the image produced by 50mm DX on the central region of the image produced by 50mm FX what will you see? Any deviations? Or a perfect match?
So far, I understand the first experiment will result in a slight deviation because one is a 75mm lens and another is a 50mm - they are two completely different things, only with a similar field of view, and so they have different perspectives.
In the second experiment, I expect a perfect match on the central region, because they have the same geometry and perspective.
Also, based on the geometry, I expect that deviation would be larger in short focal lengths (50mm and below) and they may be irrelevant on longer focal lengths (100mm and above).
Am I right ?
PS: This is a theoretical discussion about ideal lens, about geometry and perspective. Please, ignore chromatic aberrations, construction, design, makers, technology, coats, whatever.
I appreciate your patience on this subject.