I had to think about this for a while and there is more going on here than meets the eye. Maybe the nature of some of this can be proven... I came up with a simple test scenario:
- take a cam and override Exposure time, ISO and ND-Filter/Aperture
- take two DNG photos of an evenly lit surface (e.g. white screen portion, out of focus)
- 1) zoomed out
- 2) zoomed in
I took SD850 @5.8mm and 23.2mm. The lens has a printed label saying that it's aperture is 1:2.8-5.5. One would thus expect the zoomed in photo to be ~1.5 times darker in terms of linear intensities than the zoomed out. It is not. The two photos seem identical in terms of intensities reaching the sensor. They can't be.
- take exiftool and do "exiftool -BlackLevel= - ActiveArea= *.dng" on the two DNGs
- in PS levels brighten the image by factor 8 (highlight 255>31) this combs up our histogram.
We're only interested in the fully covered black sensor areas. They should be the same, no matter the zoom level. They are not as can be seen in the attached graphic. So the zoomed in photo got amplified to even out the exposure difference between zooming out and zooming in.
Now comes the interesting question. When and how was it amplified? If it was amplified after analog/digital conversion that would have combed the histogram, meaning that there should still be about the same count of different tones of noise (just broadened in terms of histogram. This would be a bad design because it would effectively lessen the bit depth of the sensor when zooming in by 1 to 2 bit. It turns out that does not seem to be happening. Counting colors on the fully masked pixels I found 133 values in the green channel when zoomed out and 282 values when zoomed in. That is impossible to do digitally.
That amplification has to be done in the analog realm prior to a/d conversion. My working theory is that there is a second factor parallel to ISO that depends on the Zoom level which gets multiplied with the amplification factor derived from the ISO setting. The two together control the analog signal amplification prior to A/D conversion. The implication of that would be that it's impossible to shoot at what equals the cam's lowest ISO when zoomed in and also impossible to use the sensor's highest analog amplification when zoomed out. In other words shot noise will increase in zoomed in photos and decrease in zoomed out ones. Can this thing whatever it is be identified / maybe even get its own override in the future?
