35mm vs 120 — what the format actually changes

The number that matters

A 35mm frame is 36×24mm. A 6×6 frame on 120 film is roughly 56×56mm. That is about 3.6× the area, and area is what carries detail and tonality — not any inherent quality of the film.

Format	Frame size	Approx. area	Frames per roll
35mm	36×24 mm	864 mm²	36
645	56×42 mm	2352 mm²	15–16
6×6	56×56 mm	3136 mm²	12
6×7	56×70 mm	3920 mm²	10

What you gain

More area means you enlarge less for the same print, so grain stays finer and tonal transitions stay smoother. It also means shallower depth of field at the same framing and aperture, because you use a longer lens to cover the same angle of view.

What you give up

Ten frames instead of thirty-six, at a higher cost per frame, in a heavier camera. Medium format slows you down — which some people count as the point.

Choosing

If you shoot in volume or need to react quickly, 35mm. If you are making prints larger than about A3 and can work deliberately, 120 earns its inconvenience.