When shooting with long exposures (multiple to tens of seconds) I noticed that in some cases colors in the resulting photos change hue. This was on Canon EOS 1100D, but I've also seen such a problem with some other cameras like Canon EOS 40D and 60Da. This even happens when only aperture is changed, without changes to shutter speed.
Below is an example pair of shots on 1100D, first with 20 s exposure, second with 30 s. All the other settings are identical (aperture f/3.5, ISO 100, focal length 18 mm). The photos were taken at almost the same time deep in the night (21:49:02 and 21:50:05 MSK on 25.02.2019 at 60°N,30°E). The images were formed from RAW files (
*.CR2), where pixel data were converted to linear sRGB using the LibRaw-provided
cam2rgb matrix, and then gamma-corrected by raising the result to the power
If you open them in two tabs and switch between them quickly, you'll notice that, aside from brightness, they also change hue: the first one is more bluish, while the second is more purplish.
To make it easier to see that the whole sky changes hue, not only near the sources of light, here's the same pair of photos, but with colors normalized so that
What is the reason for this? Is it a known problem with long exposures? How can I compensate for this to get some continuity of hue for time lapse animations where scene brightness dramatically changes in time (e.g. twilight) thus requiring different exposures? (By continuity I mean continuous change of chromaticity regardless of exposure.)