A discussion has started on another thread regarding the probable existence of a hot pixel suppression algorithm within the Sony A7S:
I now think this subject is important enough to have its own thread.
I have done some further experimentation that throws more light on this "star eating" behaviour. The evidence being seen in raw files was that bright pixels would never appear singly but always paired with an adjacent pixel (in the Bayer matrix) with the same value and same colour. The same behaviour for dim pixels - they never appear alone but always paired with an adjacent pixel (in the Bayer matrix) with the same value and same colour. This pairing of identical value pixels is very reminiscent of the infamous Nikon "star eater" behaviour. Broadly speaking, the algorithm appears to reduce the value of any bright pixel down to the maximum value of its immediate neighbours of the same colour
The experiments indicate that this hot pixel suppression algorithm is only applied in "Bulb" mode. The exposure length seems immaterial - I have seen it from 1sec up to 300sec. I haven't yet checked if all ISOs are affected - I was using ISO 2000.
Here are some test shots taken of an artificial star with a 35mm lens at F8. The images show the raw data before the Bayer matrix colour conversion has been applied.
Manual mode with 30sec exposure at ISO 2000:
Bulb mode with 30sec exposure at ISO 2000:
Bulb mode with 30sec exposure at ISO 2000 with LENR (long exposure noise reduction) switched on:
You can see that the two images using bulb mode have the bright star centre completely punched out.
Using diffraction limited optics at a faster F-ratio then the star would be tighter and would almost certainly be destroyed entirely.
This is such a disappointing result for a camera that has incredible sensitivity and performance for astro-imaging.
Edited by sharkmelley, 24 April 2015 - 07:01 PM.