This is the seventh in a series of posts on the Sony a7RIII (and a7RII, for comparison) spatial processing that is invoked when you use a shutter speed of longer than 3.2 seconds. The series starts here.
Up to now, my tests for the presence or absence of the “star-eating” Sony spatial filtering algorithm have been indirect, either using Fourier analysis or histogram plotting of dark-frame read noise to detect the presence or absence of the spatial filtering. I decided to try something more direct.
I reasoned that there are probably hot pixels in the dark-frame images that, by virtue of their great intensity and their lack of nearby bright pixels, could be considered to be virtually the same thing as one-pixel stars in actual night sky photographs. I wrote some code to analyze dark frame images, looking for pixels that were simultaneously
- a certain number of standard deviations above the mean
- having either no similarly-bright pixels as neighbors or being a certain amount over the mean of the eight neighboring pixels
As criteria for that last test, I set a parameter called starNeighborThreshold, and said that the bright pixel needed to be starNeighborTheshold times its brightest neighbor or starNeighborTheshold times the average of its neighbors. I called the number of standard deviations above the mean that were necessary for stardom (sorry, I couldn’t resist) outlierThreshold.
I ran the code against the three raw channels of the 3.2 second and 4-second exposures from the a7RII and a7RIII. Here’s how many stars I got:
You can see that, on the whole, both the a7RII and a7RIII at 4 seconds have a ravenous appetite for what I defined to be stars. However, as was predicted by Mark in a comment to an earlier post on this subject, in the green channel, the a7RIII is not quite as hungry.
Is this a fluke? I dunno. More to come.