Why don’t equal aperture/shutter stop changes give identical raw values?

Question

I photographed the same scene with both white and black areas, starting at f/22 and then opening the aperture one stop at a time while halving the exposure time for each shot. In theory, each one-stop aperture change should be canceled by the corresponding shutter-speed change, so I expected the average raw pixel values in a selected white or black region to stay constant.

However, when I measure the average raw counts in those regions across the images, they vary by about 5% standard deviation from the mean. The camera is a scientific CMOS, and I’m not intentionally changing illumination or camera position.

Should aperture and shutter speed cancel perfectly in practice, or is this amount of variation normal? What factors could cause it?

user34947 · Accepted Answer

This is normal behavior, caused by:

Imperfections of aperture. Usually there are variations from technology process which cause not to have exact size of the hole. On 50mm lens f4 you should have 12.5mm opening, but it can be 12.4mm or 12.6mm
Imperfections in shutter speed. The shutter is also mechanical unit and based on some factors as temperature, how precise are the blades and other elements inside, speed will be not 1/100s but can be 1/110s or 1/90s.
The same is true about the sensor itself (from electronic point of view)

At the end even two consecutive photos can have different (slightly) exposure.

And add fluctuation of your illumination source...

Why don’t equal aperture/shutter stop changes give identical raw values?

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