I have read that approximately a normal lens for a given sensor format is one whose focal length corresponds to the diagonal of the format, since this has a strong relation with the field of view of the human eye.
So, even if pretty much every format has a different normal lens (80mm for medium format, 50mm for 35 mm format, 28mm for 110 format for example), all of them do two things: they reproduce the field of view of the human eye and they also reproduce the relation of sizes between objects in the frame as they are seen by the human eye. Am I correct here?
When I think of a DSLR, I can't find a lens that both reproduces the field of view of the human eye AND reproduces also the relations of sizes between objects as I see them. The two options I think about are a 30mm and a 50mm lens. The 30mm reproduces the field of view correctly: I see the same field of view as if I'd have a 50mm on a 24x36mm camera. However, the relation of sizes between objects is not the same, and I can clearly see that I'm using a wide angle lens. Now, with the 50mm, the field of view is much smaller, approximately what would be the field of view of a 75-80mm lens on a 24x36mm format, but the relation of sizes between objects in the picture is just as I saw it, and just as if I had been using a 50mm on a 24x36mm camera.
When I look at different format systems, I can see that other things have been modified on them in order to reproduce the field of view and perception of the human eye -lens design, camera design, etc.-. The APS-C DSLRs are only a 24x36mm camera with a smaller sensor. When I try a 30mm lens on my camera, I can clearly see that I'm using a wide angle and not a "normal" lens. When I try a 50mm, at first glance you can say that it gives a " short telephoto lens" feeling, but none of my pictures taken with the 50mm appear as if they would have been taken with a short tele, they all indeed look like taken with a normal.
It seems that, when we're discussing which is the normal lens on an APS-C sized sensor DSLR, it comes down to what the photographers perceives as "normal", if the reproduction of the field of view, or the reproduction of the relation of sizes between objects in the frame.
How is the normal lens for an APS-C sized sensor camera determined?
Thanks in advance, this question is itching my mind and I can't seem to arrive to a good answer.