Originally posted by Class A While "f/1.8" always achieve the same "exposure" independently from the format, the total amount of light can be vastly different. The reason that an image made with a Q and an f/1.8 lens is noisier than that made with a D800 and an f/1.8 lens is because the D800 sees a much larger amount of total light. The exposure (amount of light per unit square) is identical, but there are a lot more unit squares within the sensor of the D800. That's why it makes sense to calculate the equivalent aperture of the lens on the Q which would be ~f/11. This maximum aperture of f/11 makes it obvious that the Q is not only unable to create images with shallow DOF but also that it collects a lot less light (over 5 stops less).
I am afraid that you were incorrect regarding the light advantage of FF. On FF and APS-C bodies, if you shoot at the same shutter speed and aperture, you need same ISO to achieve same exposure. You do not need higher ISO on APS-C.
FF has "better" noise at high ISO just because they have larger pixels, not because they have larger sensor. Yes FF receive more lights but the lights must be spread into a wider space, so there is no light advantages for just having bigger sensor. Giving same pixel size (i.e. D800 vs D7000, assuming there is no different about sensor technology), the crop and FF will give same result of ISO noise because each pixel receive the same amout of light. Bigger pixel also has advantages in distortion; this means you can shot at extremely small F-stops (i.e. f/16, f/22) with less blurry. Most PnS has very very small pixels, so each pixel receives very less amount of light which produces worse result.
The obvious different between FF and APS is narrower DOF which is not related to pixel size (but sensor size). That's why some review sites use "equivalent aperture in terms of DOF", they never use "equivalent apeture in terms of ISO" or something.
DOF depends on distance from camera to subject, focal length and aperture. To achieve pictures which have same angle of view with FF, on APS-C you have two options:
- Increase the distance, or
- Use shorter lens
Both options will give you wider DOF. As a result, to have the same DOF, with APS-C you need larger aperture (which result in faster shutter speed or lower ISO level to achieve same exposure).
Giving the same lens, same aperture setting, in FF you can shoot at shorter distance which give you narrow DOF, too. That is again the reason of "equivalent aperture in terms of DOF".
Normally, most PnS and smartphones never have narrow DOF because their lenses have very short focal length (the actual value, not the FF-equivalent one).
PS: I did not take circle of confusion into account (bigger sensor = bigger circle = narrower DOF). This also affects DOF only, not light advantage.