Why do Sony A7 III RAW highlights still clip at maximum values, even at base or pulled ISO?

Question

I’m trying to understand the difference between sensor full-well saturation and ADC/digital clipping on a Sony A7 III.

My expectation was that at low/base ISO (for example ISO 100 or 800), the camera should be able to accept a lot of light before the ADC clips. I also expected that if I exposed enough to trigger zebras, I might see RAW highlights flatten at some value below the file’s maximum code value rather than hard clipping to pure white.

In testing, I set zebras to 109+, adjusted exposure until only a small area zebraed, and then examined the RAW in RawTherapee and Darktable. I found some pixels sitting at a constant green value around 15860, and Darktable’s RAW clipping tool also marked some highlights as clipped.

I’m confused because I expected lower ISO to preserve more headroom, and at pulled ISO values like ISO 50 I expected the upper part of the numeric range to become unreachable. Why do the RAW highlights still appear clipped at the top end? Also, can zebras on Sony be trusted as a RAW clipping indicator?

Euri Pinhollow · Answer

if you want to be sure about what data is in raw file you should use RawDigger or dcraw with grayscale output with no processing. Ideally you should see 16384 max value which would correspond to 14 bit ADC. 15860 looks like the black level was corrected

I wouldn't expect RAW values clipped to maximum values, but to some lower value.

I am not sure you will see that in any consumer camera. I do not remember seeing non-saturated white values in any raw file I ever touched.

Cameras typically indicate clipping based on preview image which includes colour balance and colour conversion to some D65 colour space, therefore you should not rely on in-camera indication.

When I go below base ISO, let's say I go to 50, I understand it that the camera should just take whatever value ADC produces and divide it by 2, as it can't lower analog gain more.

That is indeed what can happen but it does not affect raw files, just the embedded preview and JPEG output.

not all cameras preserve raw data well. Don't know how it is today but all Nikon DSLRs were known to be mangling raw data: applying black level correction and pre-multiplying channels, making them significantly inferior for astrography in comparison.

Why do Sony A7 III RAW highlights still clip at maximum values, even at base or pulled ISO?

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