Canon ISO invariance

A lot of Nikon camera sensors are ISO invariant and unexposed images taken at ISO 100 can be recovered in post processing without any significant image degradation.

Most of the earlier Canon sensors are ISO variant and suffer from a purple colour cast when any underexposed shadows are recovered in post processing.

The Canon 5D MkIV sensor, at first glance, is also ISO variant but after testing it appears like the 5D MkIV sensor in ISO invariant from ISO 400 upwards (at least to ISO 3200) which is where I stopped testing.

Baseline exposure @ ISO 3200, f/8.0 1/50 second correctly exposed
Underexposed by 1 stop @ ISO 1600, f/8.0 1/50 second and then increased by 1 stop in Lightroom
Underexposed by 2 stops @ ISO 800, f/8.0 1/50 second and then increased by 2 stops in Lightroom
Underexposed by 3 stops @ ISO 400, f/8.0 1/50 second and then increased by 3 stops in Lightroom
Underexposed by 4 stops @ ISO 200, f/8.0 1/50 second and then increased by 4 stops in Lightroom
Underexposed by 5 stops @ ISO 100, f/8.0 1/50 second and then increased by 5 stops in Lightroom

By comparing the ‘corrected’ images above it is clear that if the exposure is correct metered at ISO 3200 and then an image is shot at ISO 400 it is possible to comfortably increase the exposure in software by at least 3 stops without any image degradation.

There is obvious image degradation at ISO 200 and ISO 100 where the purple cast starts to appear in the dark colour square and image noise increases but ISO 400 seems to be the ideal ISO to work with if you want to retain highlight details and increase the dark regions thereby maximising the dynamic range of the camera.