Why ETTR?

Started 8 months ago | Questions thread
smithling
Regular MemberPosts: 300
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Re: Why ETTR?
In reply to Ysbrand Galama, 8 months ago

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, so the end effect is the same.

That is true for cameras at certain higher ISOs. For example, on my Lumix G3, if I need to conserve shutter speed, I can underexpose using ISO 1600 (keeping a faster shutter speed) and then brighten the photo later on in post, and there will be the same noise as if I matched the camera's suggestion to use ISO 3200. Anyway, the S/N remains the same in that case for that camera. The camera is "iso-less" from 1600 on.

However, if I had ETTR by slowing the shutter instead, I would have much less noise. You can reduce the ISO, but you haven't increased the signal. Your just telling the camera to underexpose.

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

If you decrease the ISO so much that you have underexposed, you will have to brighten the photo with your raw processor (lightroom et al.) anyway to get get the required brightness. By doing so, you are increasing the noise without increasing the signal (try it). Conversely, if you ETTR, you have to darken the photo later on, which does not introduce any noise.

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