When you change shutter modes on the Sony alpha 7II, you may affect the bit depth. Because of the raw compression algorithm on this and all the other alpha 7 cameras, which is not defeatable, I need to go into a little detail about just what bit depth I’m talking about.
The raw compression scheme is to take the linear output of the analog to digital converters (ADC), which is either 12 or 13 bits, apply a nonlinear digital tone curve (kind of like gamma encoding) and resample the now-nonlinear signal at 11 bits. The camera then uses a delta modulation scheme to reduce the data necessary to encode the raw file to an average of 8 bits per pixel.
When the shutter is set to bulb*, or the shutter mode is set to continuous drive, speed-continuous drive, continuous bracketing, A7s’s silent shutter, and any possible combination of these, the linear, pre-tone-curve, bit depth of the camera is 12 bits. With all other settings, including single bracketing, the linear bit depth is 13 bits.
The loss of precision associated with the affected shutter modes is associate with an increase in the read noise by about one stop**, a decrease in the dynamic range by about a stop, and more shadow noise, as can be seen in this photon transfer curve of the a7II’s performance in the two modes:
The above curve is for ISO 100, where the effects are the greatest, since there is little analog read noise to dither the ADC.
Does this change in the photon transfer curve cause visible effects in images? I set out to learn about that.
I put a Leica 28mm f/2.8 Elmarit-R on the camera with a Novoflex adapter, mounted the camera to the RRS generic plate (the L-bracket isn’t yet available), the bracket to an Arca-Swiss C1 head, the head to a RRS tripod, aimed the camera at the bookcase I usually use for this test, set the f-stop to f/8, the shutter to 1/50 second, the ISO to 3200, the shutter to single shot, and made an exposure. Varying nothing but the ISO setting, I made five more exposures at ISO 1600, 800, 400, 200, and 100.
Then I put the shutter in continuous slow mode and did the same thing.
I developed the images in Lightroom 5.7.1 with default settings except for turning off sharpening and noise reduction. Then I set the Exposure control on the ISO 100 image to an extra +5, the Exposure control on the ISO 200 image to an extra +4, the Exposure control on the ISO 400 image to an extra +3, the Exposure control on the ISO 800 image to an extra +2, the Exposure control on the ISO 1600 image to an extra +1, and didn’t adjust the Exposure of the ISO 3200 image.
I cropped the images and magnified them by a factor of two.
There is quite a bit more noise in the continuous case.
The single shot is better, but not by much. The extra noise is providing useful dither.
I don’t see much difference at all.
The ISO 800 and 1600 exposures continue the trend. Here are the ISO 3200 exposures:
No difference. By the way, don’t read too much into the color balance differences in the above images. I left the WB in Lr set to “As shot”. My bad.
OK, what if we do the same series, but this time start at ISO 12800 and go to ISO 400, with the shutter set to 1/200. This time I’ll make the WB the same.
Here are the five stop pushed images:
There’s not as much difference as in a five stop push at lower ISO, but the single shot image is still a bit better.
*By the way, you should know that, at least on the a7S and a7II, bulb mode invokes some spatial filtering. Oh, those tricky Sony engineers!
**The drop in precision to 12 bits is not the sole cause of the increase in read noise. You can see what simply lowering the precision to 12 bits does in the Nikon D810, which has a mode that effects the change from 14 to 12 bits through truncation.