[alxv]

The 35mm format is rather amazing balance of design trade-offs. No wonder it is so enduring.

It's large enough for the double-Gauss lens (a.k.a. normal prime lens) to have a nice shallow deep of field wide open. f/1.2 is close to the limit of a typical SLR mount. So a normal 50mm f/1.2 lens gets us 42mm of aperture. This means we can get the same depth of field and angle of view as a 6x7 medium format with a 110mm f/2.8 lens or a 4x5 large format with a 180mm f/4.5 (but with a much smaller system!) And we get a faster lens as a bonus.

Smaller formats lose some of that versatility of composition. The focal length of the normal lens on APS-C is 32mm. We would need f/0.8 lens to the same DOF. That's not possible on an SLR mount. Mirrorless systems with their shorter flange distances could get us there. But even then there are limits to how short the flange can be because image sensors become much less efficient as the angle of incidence of the light hitting them increases.