Why ETTR?

Started 9 months ago | Questions thread
Allan Olesen
Senior MemberPosts: 2,255
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

Ysbrand Galama wrote:

 A lower ISO has a better S/N-ratio, so the end effect is the same.

No. At the same amount of light, a lower ISO will sometimes have a worse S/N-ratio and sometimes an unchanged S/N-ratio, but almost never will it have a better S/N-ratio.

(I write "almost" because I have heard of exactly one camera model where the S/N-ratio is actually better at lower ISO with the same amount of light.)

If you don't believe me, try to take a raw photo at ISO3200 with the aperture and shutter speed appropriate for ISO 3200. Then take another raw photo at ISO 100 with the same shutter speed and aperture and increase the brightness in your raw converter 5 stops so the two photos are equally bright. If you have an ISO-less camera, the noise will be the same. But if your camera is not ISO-less, the ISO3200 will have less noise.

A high ISO is not a cause of noise. It is only a symptom, showing that the amount of light is too low. So you should give the variables which affect the amount of light (aperture, shutter speed and scene lighting) much more attention before you even begin thinking about ISO.

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