ISO and signal to noise ratios Locked

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Auf Reisen Regular Member • Posts: 116
ISO and signal to noise ratios

In the recent Sony interview here on DPReview, the author said

While this a7R II certainly did the job perfectly well capturing this high dynamic range scene on my recent trip to Iceland, a lower base ISO that would have allowed me to give the sensor even more exposure before clipping the highlights to the left, yielding an even cleaner image.

I am not sure I understand the logic behind that statement. Bear with me, I am thinking aloud here. If I understand correctly, a higher ISO means that, for any given amount of light that falls on a sensor pixel, this will be translated by the imaging software into a brighter image pixel in the image file. As far as I understand it, the usual linear relationship between ISO and signal/noise ratio is the wrong way to think about it. What we perceive as, say a uniform blue sky, is in reality the result of many different photons with different wavelength hitting the medium (film, sensor, eye), the average of which is perceived as 'blue'. The less light is gathered, the higher random outliers factor in, and the lower the signal to noise ratio.

BUT: signal/noise is a matter of how much light is gathered, not about how it is translated into brightness by the image processing software (well, it is, but this is secondary). So a picture taken at f/2.0 1/800 at ISO 200 has the same signal/noise ratio as taken at f/2.0 1/800 at ISO 400, with the latter just appearing brighter. How bright is bright enough of course depends on the scene.
So, for any given scene at a fixed brightness level, lowering the iso means I can afford to let more light in by stopping up or prolonging the exposure, which in turn means a better signal to noise ratio.
Is that about correct?

 Auf Reisen's gear list:Auf Reisen's gear list
Olympus OM-D E-M10 II Panasonic Lumix G 20mm F1.7 ASPH Olympus M.Zuiko Digital ED 40-150mm 1:4-5.6 R Tamron 14-150mm F/3.5-5.8 Di III
Sony Alpha a7R II
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