Originally Posted by Jon Ruyle
The reasons used to support the 'less light with smaller sensor' above are all dependent on the characteristics (not just size) of the sensor itself, not the amount light falling on it. Yes, croppingmakes pictures appear more noisy - it doesn't make then noisier, the noise is there (digitally), but enlarging them makes it more visible in the image. Smaller sensors have more noise, but that depends on the 'all other things being equal' clause - smaller pixels have more noise, yes, because their photon depth is shallower (lower eV) meaning they have a lower signal to noise ratio. But, if you pack 4 'shallow' pixels into the same physical area as one 'deep' pixel, you'll capture nearly the same amount amount of light (some is lost due to inter-pixel spacing), with a tradeoff of more noise and less dynamic range. Because all of these are dependent on sensor properties, not size, the 'crop factor' light loss is not going to be 1.6 here. These same issues will affect light gathering ability across the board - some FF sensors will gather more light than other FF sensors, and there are newer crop sensors that will gather more total light than older FF sensors. This is due to technological improvements, as evident from the higher ISO values achieved in newer sensors (of all sizes), even in the face of reduced pixel sizes - the 'wells' are getting deeper. It doesn't make sense (to me) to account for these pixel differences with a fixed sensor-size crop factor - you'd have to apply a correction like that when using the same lens on an old 1Ds MkI vs. the forthcoming iDs MkIV, or a 50D vs. a Rebel XT, etc., as the equal-sized sensors in each pair have very different light-gathering ability.
It makes sense that image sensor size is not an issue for AF points - the AF system uses a different sensor, and that sensor is still seeing an 'f/2.8' amount of light, because that's what is being delivered by the lens - to both sensors. That's also why the 1D series can autofocus at f/8 vs. f/5.6 like all other bodies - it has a more sensitive AF sensor.