Sony A7III Posterization and Colored Banding

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sharkmelley
sharkmelley Senior Member • Posts: 1,710
Sony A7III Posterization and Colored Banding
5

I know there have been many recent threads on coloured banding issues. They typically result in heated discussion but we still don't fully understand the underlying cause(s). The purpose of this thread is to start digging down into this issue and gain some understanding. I'll begin by repeating some analysis that might otherwise be lost in the middle of this thread Canon EOS R -I did it - Continued

In that thread mcslck uploaded some "white wall" examples. The link to the raw files can be found in this post: https://www.dpreview.com/forums/post/62001143

Although images of white walls don't hold much aesthetic interest, they are much easier to understand and analyse than discolouration of suits at a wedding!

So here is one of those white wall images which I have colour saturated to death. Note that in-camera lens vignetting corrections were OFF:

Looking in the EXIF we can see that the camera firmware supports the lens, because the EXIF contains the following entries:

Distortion Corr Params : -6 -1 5 11 19 27 36 44 51 54 53 47 34 10 -27 -82
Vignetting Corr Params : 0 32 96 224 384 576 864 1248 1728 2368 3200 4256 5472 6816 8224 9632
Chromatic Aberration Corr Params : 310 314 316 314 310 308 292 236 166 92 32 24 26 16 -26 -60 -417 -414 -405 -389 -368 -336 -306 -294 -285 -265 -242 -213 -192 -188 -212 -188

One possibility is that the camera is still performing some kind correction even though the vignetting correction is off. Such behaviour has been previously suspected with a different model: the A7RII.

But there is another interesting feature I discovered in the RawDigger histogram:

Sony A7III histogram gaps

There are regular gaps in the histogram. They are spaced precisely 129 units apart. A possible explanation for this is that a scalar multiplier slightly greater than 1.0 has been applied to the digital data.

The problem with applying a scalar multiplier to the data is that it leads to regular colour shifts in the data that correspond to each gap in the histogram. It was discussed at length here:

https://www.dpreview.com/forums/post/59378650

These colour shifts show up especially in the shadow areas where they can lead to quite material colour changes. They are most often noticed when the shadows are pushed in post-processing. The other place they can can show up is in slowly changing gradients such as a blue sky background.

The above data was shot using A7III firmware version 2.0 but I've looked at a few A7III version 1.0 images and they show exactly the same gaps.

So we now have two plausible explanations for these concentric coloured bands:

  • Scaling of the raw data by a multiplier
  • Lens vignetting corrections

Since the histogram gaps seem to be in every image, I think it is quite likely that some images contain colour banding and or discolouration caused by the two effects acting simultaneously.

Mark

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Takahashi Epsilon 180ED
H-alpha modified Sony A7S
http://www.markshelley.co.uk/Astronomy/

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