Started Dec 20, 2013 | Questions thread
texinwien Veteran Member • Posts: 3,326
You're still missing something

Ysbrand Galama wrote:

I am still missing something. The purpose of ETTR is to improve the signal to noise ratio, so increasing the exposure time makes sense, I understand that.

But if you want to improve the S/N-ratio, why not reduce the ISO instead of lengthening the exposure time? A lower ISO has a better S/N-ratio

No, it absolutely does not. ISO is not a part of the exposure equation. Shutter speed, aperture and scene brightness are the only three variables in the exposure equation. If you hold those three variables constant and lower ISO, you will not have an improved SNR.

so the end effect is the same.

Or, worded differently: when your lense is wide-open you have two ways to increase the S/N-ratio: increase shutter time, or reduce ISO.

Wrong. The two ways to increase SNR in your scenario are to increase shutter time or increase scene brightness (e.g. with a flash). Reducing ISO won't increase SNR.

Why is increasing the shutter time better than reducing the ISO?

Because increasing shutter time raises the amount of exposure while reducing ISO does not.

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