Originally posted by falconeye A certain increase in ISO performance is required for equivalent DoF with a larger sensor, correct, and exactly this required increase is delivered by the larger sensor. It mathematically cancels out.
I had to think about this for a moment, but of course, if the same shutter speed and DOF (aperture) is used, the same amount of light will hit both big and small sensor. While the ratio light/area is worse for the big sensor, it also has more area to make up for it.
So Joseph James is wrong when he writes "... if we were to try to achieve an equivalent image from, say, a Fuji f31 at f/2.8, 1/50, ISO 1600 using a Canon 5D, we would have to use f/13, 1/50, ISO 33000." or is ISO sensitivity somehow specified in terms of a light/area ratio? Even though, the higher ISO number for a FF sensor would be kind of misleading then and comparing ISO numbers directly would be as invalid as comparing f-stops (f-ratios) directly.
Thanks a lot for pointing out the forces regarding secondary pixel size effects. Do you have a pointer to something that explains why read-out noise is lower with higher pixel density? In principle, the lower the signal, the higher any electronic noise floor should become in proportion.