After completing exercise 1.1, I was intrigued about whether the histogram variations were solely related to scene variations or were caused by the camera, either as digital noise or vagaries in the light metering. I decided to find out by repeating the exercise while taking control of as many variables as possible.
These images are shot indoors at night (to eliminate daylight variations) lit by a single 500w photoflood lamp for consistent lighting (flash might vary between exposures). ISO was set manually at 100, to minimise noise. The camera was tripod-mounted and image-stabilisation was switched off.
In this first set, I used programme-automatic metering and automatic white balance. The resulting images are identical at a ‘collect-and-jitter’ level, so I have only shown one here.
These histograms are much more consistent than in the previous (outdoor) exercise. Collect-and-jitter showed the three main peaks shifting slightly. Closer examination shows that the moves are no more than one pixel to left or right.
In the second set, I adopted manual exposure control and a custom white balance (3000K as recommended for photofloods) The only variable remaining should be digital noise. Again, the images were identical to each other at a ‘collect-and-jitter’ level.
These histograms are almost identical. Collect-and-jitter shows no left-right shifts and a slight variation in overall height, possibly a vagary of the histogram-drawing routine. Viewing the image above shows slight variations at a single-pixel level.
My conclusion is that a very small part of the histogram variation seen in the original exercise is camera-generated, but the majority relates to the very small variations in that scene.