What's this??!! ( circular ring artifacts )

Started Feb 26, 2014 | Discussions thread
mbietenholz Regular Member • Posts: 101
Re: What's this??!! ( circular ring artifacts )

wictred wrote:

mbietenholz wrote:

I'll try this later when I get a chance. I don't get it - turning the vignetting correction on in the camera does *not* cause the vignetting correction to be baked into the RAW data, but *does* cause some other subtle change in the RAW data that produces banding at very low light levels?


But it is only a very very small effect ...

However when it is the lower bits of the RAW it will become visible when you push the exposure afterwards.

The shadows don't contain that much detail, and if you "amplify" the result in PP, small differences become visible. Just like shadow noise becomes visible the same way.

I stand corrected: you are indeed right wictred - the setting of vignetting-corrections (Lens Comp: shading) *does* affect the RAW data for the NEX-6, at least for the 16-50 kit lens (SELP1650). I just did some test shots, and it looks like turning this correction on in camera causes a partial correction for vignetting to be made to be baked into the RAW data, with the side effect that banding can be produced at very low light-levels.

My guess is the banding is likely the result of doing integer math when doing the in-camera vignetting correction.  It seems unlikely to be due to the raw compression used because that would affect the image whether the in-camera correction was enabled or not.  Also, the raw-compression is pretty local so I don't think it would produce this kind of effect

As Wictred said, the banding only shows up at very severe underexposure, like -4 or more stops, so the banding is probably a pretty minor issue in real life.  I will continue to leave the vignetting correction enabled in-camera - that way the OOC jpegs will be good to go for those shots where I don't want to bother with raw conversions.  If, however, you plan on digging way down into the noise, you might want to disable this correction in the camera (astrophotography is one possible case where you may to disable it).

It may also not affect all lenses -  the SELP1650 kit-lens has fairly severe vignetting issues, so the correction for vignetting is fairly strong (especially if you *don't* correct for the lens distortion).

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