Sorry for the perhaps controverisial heading, but I thought that it might catch the attention, as well as it does convey something that, at least to me, came somewhat as a surprise.
When considering FF vs APS-C I always took as absolute fact that if all things equal, the FF will have about one stop better low light performance. This based on that with double the area, it will have one stop better ISO performance.
But then I started to think what parameters I play with when constructing an image. The parameters I control are FOV, DOF (aperture) and shutter time. The ISO setting comes as a consequence of these parameters (and sometimes as a limitation if noise becomes too high).
So then I thought that in order to have more of an "apple to apple" comparison, I should compare the noise performance under the same image parameters, i.e. under the same FOV, DOF and shutter time.
For a given FOV and DOF, the required f-number is proportional to the format size. Thus with FOV, DOF and same shutter time, the FF will require a one stop higher ISO compared to the APS-C.
In order to see how a SNR curve between APS-C and FF would look like if comparing images taken under the same image conditions (same FOV, same DOF and same shutter speed), I took the DxOMark numbers for Nikon D90 and D700 (I used these cameras since they have the same pixel count and was released roughly at the same time, so they should be similar in technology evolution status, but I do not want to trigger a brand vs brand discussion).
I have attached the curve I ended up with. Of course, the comparison holds only within the limits that the aperture of the lens can be changed.
A very practical situation would be e.g. in sports or in nature shooting using long focal length lenses. Using a lens of 400mm f/2.8 for APS-C and 600mm f/4.0 would actually result in images having the same noise performance. But the 600mm lens would set one back an additional $2000 dollars (using Nikon prices)
I am sorry if I have stated something that is old news to you all, but too me the result came somewhat as a surprise and was not what I had intuitively thought before. If I have made any logical sommersaults in my thinking, I will be happy to be corrected. As for now, it made me realize that for the way I shoot images, a FF would actually not give me any benefit in terms of low noise performance.
Best regards,
Haakan