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p.1 #1 · p.1 #1 · Latest A7R firmware (3.3) no longer delivering 'raw' raw on long exposures | |
A recent post on SonyAlphaRumours available here, reports that the latest version of the A7RII and A7SII firmware (3.3) applies some form of median filtering of the raw sensor data on all exposures longer than 3.2 seconds, with these changes then locked irrevocably into the raw file. A number of sources provided in that post point to different demonstrations of this effect, which in astrophotography circles is generally referred to as a 'star-eating algorithm'. The reported significance of this change is that it reduces substantially the luminance of small stars, leading to significant loss of night-sky detail in any exposure longer than 3.4 seconds.
As someone with an occasional interest in astrophotography, I decided that I should check out the magnitude of this, taking two sets of images, controlling as carefully as I could to eliminate any other sources of variability. I took two sets of images of part of the Milky Way with my ZM Distagon 35mm at F/2.8. I shot thirty frames for each set, with one set using exposures of 3.2 seconds (i.e., below the exposure threshold at which filtering is applied) and a second at 6 seconds (above the filtering threshold described in the Sony Forum post). I used Sony's timelapse app with an ISO of 6400, silent shutter and with a 1 second gap between each exposure. The only difference in setting between these two sets of images was the exposure time.
I processed these images in three steps as follows:
1. I initially processed the images in Lightroom applying very minimal settings, i.e., manually setting the colour temperature, setting the black slider to -80 to compensate for the light (urban-lit) sky background, and setting the exposure slider on the 6-second exposure set to -0.91 to equalise its exposure relative to that of the 3.2 second set. All other sliders were set to zero except for the saturation slider, which was set to 5.
2. I exported a 2000 by 2000 pixel subset from both sets of images, and aligned each set in DeepSkyStacker, using default star-detection settings, and saving the aligned images. Interestingly at this stage DeepSkyStacker detected around 21,000 stars per image on the 3.2 second set, but only 6100 stars per image on the 6 second set.
3. Finally, I separately imported the two sets of aligned images into Photoshop using the script 'load files into stack'. I combined all images in each set by changing the opacity of the 2nd to 28th images in the stack to 4%, but the top two images were both set to screen mode to brighten the image. I then flattened each and saved the images as single tiffs. In effect this largely emulates the process applied in Sony's smooth reflection app, where multiple images are combined by averaging to create a single image with greatly improved signal to noise ratio.
While the pair of final images superficially appear similar as in the first image below, zooming in to a higher resolution (300%) as in the second image reveals a quite different picture. IMHO the second image pair clearly demonstrates a loss of fine-scale detail occurring presumably as a consequence of this additional 'filtering' step implemented by Sony in their latest firmware for exposures longer than 3.2 seconds.
Several of us have tried to raise this issue with Sony on their community forum, but so far haven't managed to generate any great engagement beyond a generic 'yes we value your opinions'. My own view is that I would like Sony to either reverse this imposition of an arbitrary data filter step on longer-exposure data, or to provide an option to save the pure raw format data in a strict sense.
However, I would be interested in the views of others as to (i) the robustness of the comparative test procedure that I've used, (ii) the significance of the differences I've demonstrated, (iii) whether you see this as having implications in your practice of photography with an A7 series camera, which might include use of long exposures for subjects other than stars, and (iv) whether you have any suggestions on how to encourage Sony to engage positively around this question to restore access to the full sensor functionality that we enjoyed prior to this latest firmware 'update'.
Any other practical suggestions welcome...
-John
Thirty stacked and aligned images - left at 3.2 seconds/image - right at 6 seconds/image
Same image pair at 300%
Edited on Apr 29, 2017 at 02:59 AM · View previous versions
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