Sony FF Extended ISO; DR Implications

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SilvanBromide Senior Member • Posts: 1,630
Sony FF Extended ISO; DR Implications
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Earlier this year, The Northrups posted a video appearing to challenge common recommendations to disregard extended ISO settings, offering some example images to illustrate purported noise benefits at ISO 50.

FWIW, the video is at:

https://www.youtube.com/watch?v=WAQkOQQQJP4

I don't think Tony explains his point well - some of his language is unclear (or at least, his explanation seems incomplete). Curious what people's take on this is.

If there is nevertheless a semi-plausible explanation for the effect he seems to demonstrate, it's that the sensor read data from base ISO is essentially being scaled when in extended ISO, to lower the apparent sensitivity. With ISO 100 scaled to ISO 50, scaling halves the noise along with the image data collected by the sensor. If you were then able to add a stop of exposure time or widen the aperture (without clipping) the amount of light captured would be increased WRT noise, for a net gain in DR. But...

The flaw in this line of thinking seems to be that FWC of the sensor does not change, so it seems doubtful that you can expose (larger aperture or lower shutter speed) at ISO 50 beyond the level you'd have used at ISO 100 - assuming you were already exposing for the highlights (or else the highlights will clip in the resulting image...). IOW, if the clipping point is scaled along with the noise, you gain nothing.

That being the case, Tony N's point seems mostly moot. That is, it's only if you weren't following ETTR rule-of-thumb in the first place that you'd see the benefit he claims. In which case, the images he uses for illustration seem misleading. I.e. if he was exposing for the highlights in both cases, there would be no perceptible difference...

Am I mistaken? What's other people's take on this?

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Former Canon, Nikon and Pentax user.
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