This is a continuation of a series of posts on the Sony a7RIII. You should be able to find all the posts about that camera in the Category List on the right sidebar, below the Articles widget. There’s a drop-down menu there that you can use to get to all the posts in this series. You can also click on the “a7RIII” link in the “You are here” line at the top of this page.
Truth in advertising: this post is more about the a7RII than the a7RIII, although the newer camera does play a part at the end — here’s a bit of foreshadowing shamelessly designed to keep you reading — ending the post on a downer.
But first, the good news. The a7RII star-eating spatial filtering that takes place at 4 seconds and longer exposures does not occur in continuous low or continuous high shutter modes.
I will show you histograms and spatial frequency plots that are all derived from dark-field (back of body cap) photographs at ISO 640 and 4 seconds, saved in uncompressed raw.
The first is the frequency plot for a single shot shutter mode a7RII (FW 4.00) image with which frequent readers of this blog are all too familiar:
The attenuation in both directions that takes place at higher spatial frequencies is the smoking gun for star-eating.
But if we set the shutter mode to continuous low, we see this:
The periodic behavior in the horizontal direction is indicative of some kind of processing, or some sensor strangeness that happens when the ADCs are run fast, but it’s not star-eating. The vertical direction looks flat as a pancake.
Things look about the same in continuous high shutter mode:
What’s going on?
A look at the histograms provides a clue. First a single shot image:
The green channel has a little ugly combing, but all 14 bits are present and accounted for, and the noise is pretty low.
In continuous low:
The analog to digital converter (ADC) has dropped back to 12-bit precision, and the read noise has increased somewhat.
In continuous high mode:
Looks about the same.
So, the good news is that you don’t have to live with the star munching if you have an a7RII and are a serious astrophotographer. The bad news is that you have to put up with a little more noise and 12-bit precision. As bad news goes, that’s pretty good. The increase in noise is small, and the read noise at ISO 640 provides sufficient dither that you won’t miss those missing bits (at higher ISOs there will be even more read noise, and the increaded noise from the 12-bit conversions will be less significant). There is some question about what that horizontal periodicity means for astrophotography, and I would be interested in hearing from people who use this workaround on real stars.
What does all this mean for a7RIII users? Unfortunately, it means nothing.
Here’s a single shot image from an a7RIII under the same conditions:
Some old ugly behavior.
Now in continuous low:
And in continuous high:
Same old same old.
We can get an idea of why by looking at the histograms. First, single shot:
Like the a7RII one, but the distribution is more ideal.
Now continuous low:
Aha! The ADC is not dropping back to 12 bits.
So the a7RIII is faster than the a7RII, and doesn’t need to run its ADCs at lower precision in continuous mode. Presumably, it also has time to do the spatial filtering, too.
Win some, lose some.