How can an electronic shutter reduce dynamic range?

Question

I read a recent review of a camera which can use either its mechanical shutter or an electronic shutter. The review mentions a problem they noticed with the mechanical shutter, and then notes that the problem "can be eliminated by switching to electronic shutter mode, though this comes with a slight cost to dynamic range and a risk of rolling shutter."

I understand the rolling shutter issue, but am curious as to how the electronic shutter might affect dynamic range. Several answers to Why don't they make all DSLRs with an electronic shutter? suggest that this is because of the sensor construction required to enable such a shutter. But that doesn't make sense in this context — if it were a limitation due to construction, the dynamic range would be reduced even if that part is switched off and the mechanical shutter used instead.

So, what's going on here? Is the review simply carrying the knowledge that electronic shutters increase noise without understanding the details, or is there some other reason electronic shutters increase noise?

Answer 1

The review has pictures to confirm higher noise of electronic shutter at similar settings, so it's not just a matter of prejudice.

On some recent models, e.g. GH4 and GM1, Panasonic has been caught using reduced bit depth (10 bits instead of 12) with electronic shutter to speed it up. As a half-price alternative to GH4, I'd expect G7 to pull the same trick, too. Less bits can express less stops of light, so it'd make a valid cause for reduced dynamic range. This would also nicely explain why lifting exposure in post-processing shows more noise in e-shutter images than mechanical shutter images, while unlifted ones look rather similar.