Originally posted by clackers If 9000 photons fall on a pixel, then 9000 photons also fall on the pixel in APS-C mode.
Yes, but with 36M collectors that collect 9k photons each, you will collect 2,25 times as many photons as those 16M collectors that collect 9k photons each.
Assuming you use the same focal length too you will get the same light and noise per detail in the image, but a l larger image. If you stand 1 meter from both the 2,25 meter wide image and the 1 meter wide cropped image, you will see the same amount of noise per detail. But if you do the more natural thin, take two steps back to 2,25m distance, you will get less noise per image with.
If you do change focal length according to the crop factor, view the images in equal size from equal distance, you will get less noise per detail in the image from the FF camera, using the same aperture number. The reason for this that aperture
area changes with physical focal length, when the aperture
number are held constant.