Why ETTR?

Started Dec 20, 2013 | Questions thread
gollywop
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Re: Why ETTR?
In reply to Ysbrand Galama, Dec 23, 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.

I'm going to assume you're shooting raw. The first thing to note is that ISO has nothing to do with exposure, so your second sentence in the last paragraph above is meaningless. The third sentence is even more so. ETTR is always assessed at base ISO. ETTR is increasing exposure (aperture and SS) to the point where the (raw) histogram is pushed just up to its right-hand edge -- at base ISO.

The second thing to note is that your appropriate exposure is not what your camera "measures," it is what aperture and shutter speed are required for your shooting conditions. Your required aperture is dependent upon the DoF you desire. If you are looking for a relatively shallow DoF, then, perhaps f/2 is correct, or some other lower f-ratio. If you're looking for a wider DoF, then a higher valued f-ratio is more likely appropriate.

The desired shutter speed is dependent upon camera-shake and motion-blur issues. If you've got decent IS or IBIS, you might be able to sustain fairly long exposures, even, say to 1/25 secs. Otherwise you'll need shorter exposures, say, 1/60 secs. or shorter. Motion-blur requirements can dictate other shutter speeds.

Now, if at whatever shooting parameters you determine, your exposure (at base ISO) still doesn't produce ETTR, then you must either compromise on (a) f-ratio (if possible) or (b) shutter speed (if possible) or (c ) some additional brightening (not exposure). If you opt for (c ), then you must decide where the brightening is to take place, either in camera via the ISO control (a reasonable option if you're shooting jpeg) or in your raw converter during processing. Just where that is best done depends on the ISO-nature of your camera.

If your camera is ISO-invariant, the brightening can be done in with the raw converter. If it is ISO-variant, then you're better off doing it with the camera's ISO control (being careful to avoid over brightening and blowing desired highlights). And if your camera is partially-ISO-invariant, then you are best off using the camera's ISO control up to the ISO-variant limit and doing the rest in the raw converter.

It should also be noted that, if you are able to achieve ETTR (at base ISO) and the image is too bright, it is not over-exposed as some posters have indicated; it is simply over-brightened but able to have its brightening properly reduced during processing -- with the benefit of increased s/n because you've used the maximal exposure (without over exposing).

Another thing to be noted is that increasing ISO does not, as a practical matter, add noise, but rather merely amplifies existing noise. Indeed with an ISO-variant camera, increasing ISO actually reduces the read noise.

Why would ISO 1600 give better results than ISO 800? I maybe exposing to the right, but also the sensor noise increases, negating the effect.

I must be missing something.

Y.

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gollywop

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