Why ETTR?

Started Dec 20, 2013 | Questions thread
Anders W
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Re: Why ETTR?
In reply to Ysbrand Galama, Dec 21, 2013

Ysbrand Galama wrote:

Can someone explain to me why I should expose to the right (ETTR) in low light conditions.

Let's take an example to explain what I do not understand. Let's say my camera measures F2.0, 1/50th and ISO 800 (no over- or under exposure). I want to ETTR so I set my camera to F2.0, 1/50th and ISO 1600.

Why would ISO 1600 give better results than ISO 800?

It wouldn't, unless the read noise (as measured in electrons) is lower at ISO 1600 than at ISO 800. Whether that is the case or not depends on exactly what camera/sensor you are using.

Consequently, it doesn't always make sense to ETTR at higher ISOs. For so-called ISO-less sensors (those whose read noise is as measured in electrons is pretty much constant across the ISO range), it does not make sense at all. For other sensors, it makes sense up to a point across the ISO range (the point at which the read noise stops falling) but typically not at very high ISOs.

Provided that there is no read-noise advantage associated with cranking up the camera ISO, you are actually better off staying at a lower ISO since that will get you more highlight headroom (i.e., delay the point at which the highlights start clipping).

I maybe exposing to the right, but also the sensor noise increases, negating the effect.

It's not really that the sensor noise increases. It's rather that there is no improvement with regard to photon noise since the exposure is the same and no improvement with regard to read noise, unless the sensor is such that the read noise (as measured in electrons is reduced at higher ISOs).

I must be missing something.

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