Sony a7S read noise in silent mode

The Sony a7S is unique among the alpha 7 series in that it has a fully electronic shutter option, called “silent mode”.

Here’s the read noise histogram at ISO 400K with the shutter in silent mode and the drive mode set to single shot:

The histogram is, to put it kindly, sparsely populated.

At ISO 50K, it looks like this:

You can make out the generally-Gaussian density function, but there is severe combing.

At ISO 25K:

Now the combing stops except for the 13th and 14th bits being MIA, and that funny drop out on the left side of the peak.

At ISO 3200:

This histogram is tightening up nicely.

At ISO 1600:

The broadening effect that we saw in single shot mode doesn’t happen here.

At ISO 800:

Now we get the broadening.

At ISO 100:

The histogram is pretty tight, but remember we’re missing the two LSB’s.