Nikon DSLRs have three image sizes Large/Medium/Small. For instance, the D800/E has 36Mpix, 20Mpix, and 9Mpix. How does the camera arrive at the pixel values for these sizes? 36Mpix obviously uses the values for each individual pixel that the sensor has. But what about the 20 and 9 Mpix? Does the camera average the pixels to create the values for the smaller number of pixels when shooting Medium or Small? For instance, does it average 4 of the 36Mpix to create one of the 9Mpix when you are shooting in Small?
I ask in particular, because if the pixels are actually averaged, the signal/noise level at 9Mpix would be higher, due to the averaging, but of course the resolution would be less.