Scenario 1: Normalize for pixel size
We have a camera that we can swap sensors on. We mount a 100mm lens to that camera.
The balance you select between the three is what determines the performance of your camera.
The only thing I would dispute is the regular use of "each sensor will have the exact same IQ". Hate to say it, but that is wrong...at least, so long as another required factor is not specified. Sensor size is the primary factor that controls "image quality", not pixel count, not pixel size. You could have twice the Q.E. on a smaller sensor, and the only one that would then only EQUAL the IQ of the larger sensor is the one that is exactly half it's size. A 4/3rds or 1/2.3 could never compare to the IQ of a FF sensor, not even with double the Q.E.
Now, the statement about equal IQ would be true...IF the aperture was specified. The 100mm lens on a FF sensor has to be using a smaller aperture than the 61.7mm lens, by a factor of the differences of the sensor diagonals. If the 61.7mm lens is f/2.8, then the 100mm lens should be f/4.5. THEN, and ONLY THEN, would "each sensor have the exact same IQ." You have to use a smaller aperture to normalize the amount of light reaching the sensor. Otherwise, one has to assume the same aperture. A 100mm lens on FF produces the same FoV as a 61.7mm lens on APS-C, however if they are both f/2.8, the FF sensor is without question gathering more light.
Equivalence. Aperture matters here.
I've said this so man times...but, I think Orangutan is the only one who actually heard it. So I'll just quote his answer:
I am sorry but neither you nor Orangutan are following Don's logic, and are traversing an entirely parallel path. It is funny, because no one is actually disputing anyone, but saying the other is wrong.
1. Forget aperture (or presume DoF to be infinity for comparison purposes)- focus on the question at hand: if the pixel size is the same, with same sensor tech, an APS-C sensor will have same noise characteristics as a FF sensor. Example: DX mode in a Nikon FF dSLR- what happens there is that a portion of the sensor is being used. Exactly the same thing will happen if you have a smaller sensor with same pixel size.
2. I don't think Don or anyone is claiming that a smaller sensor can collect more light than a larger one, if the pixel size is larger in the former. We all know that is quite impossible. The point being made is that the amount of light per pixel depends on the size of the pixel- in case of a larger sensor it is additionally multiplied by more number of pixels. However, if a FF sensor was made, using the same technology as a 7D sensor and with the same pixel size (making it 46MP I think) then it would be equally noisy.
I am not arguing that aperture affects image quality, but for this discussion let's just talk noise characteristics.
This is EXACTLY CORRECT. The only thing that really matters in the end, assuming all the sensors use the same technology, is TOTAL light gathered. More light, less noise. It's as simple as that. Pixel size doesn't really matter from a noise standpoint. Smaller pixels in the SAME sensor size mean you get more resolution, that does increase IQ...but smaller pixels in a smaller sensor DO NOT mean better IQ....they just mean more resolution, but with worse IQ.
This is equivalence. This scientific concept is documented very, very, very thouroughly here:
If you still doubt, please, read the article on equivalence linked above. It's really not that complicated of a concept.
Sorry, but that's only partially true. The pixel size does matter. Quoting from your own link:
Given four cameras, one with...
...an mFT (4/3) sensor,
...another with a 1.6x sensor,
...another with a 1.5x sensor,
...and another with a FF sensor...
...a photo of a scene from the same position with the same focal point and the same settings (e.g. 25mm f/1.4 1/200 ISO 400) with all cameras,
...the photos cropped to the same framing as the photo from the mFT (4/3) camera,
...and the photos are displayed at the same size...
...then the resulting photos will be Equivalent. In addition, if...
...all the sensors are equally efficient, then all the photos will also have the same noise,
...the pixels are all the same size, the AA filter the same strength, and the lens is the same sharpness, then all the photos will also have the same detail,
...the exact same lens is used and the sensors are of the exact same design with the exact same size pixels, AA filter, CFA, and processing...
It is possible to have same noise characteristics if the pixel size and sensor tech is the same, irrespective of total sensor size.