Anybody here using base-ISO exclusively or predominantly?

Started 10 months ago | Discussions thread
Maurizio De Cecco
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Re: interesting question
In reply to dmaclau, 10 months ago

I think there is a basic misunderstanding of what technically speaking ETTR is about.

The point is not about the sensor; ETTR will not do anything to the noise produced by the sensor (because you amplify it when you raise the ISO); and as the OP said, with respect to the sensor noise, ETTR is interesting only at base ISO.

But this is not the point. The point about ETTR, that is an universally valid point (but this do not make it more or less useful) is about reducing the artefacts of the analog to digital conversion that happens after the sensor.

In a complex system, where a signal flow from an element to another, each element have its own dynamic characteristics; the goal is to keep all the interesting information moving from one element to the other, i.e. to keep the signal within the dynamic range of the elements of the chain. In the audio world, this is called managing the chain structure.

Assuming that the ISO changes affect the analog circuits *before* the analog to digital conversion, changing the ISO, for a given exposition, change the positioning of the photo dynamic range with respect to the A/D converter. Too high, and highlights will not be recoverable. Too low, and the artefacts of the conversion will be more visible, because you will not effectively use the A/D resolution (let's say 12 bit), but less, a bit or a lot less.

For this reason, since (and supposing that) ISO change is after the sensor, changing ISO do not ajust how the dynamic range of the sensor is used with respect to the available light; the only thing you can do is adjust exposition to maximize the use of the sensor and reducing the sensor noise to light signal ratio.

The artefacts of digital conversion are fondamentally noise; how visible depends on the algorithms the camera use; also, the conversion resolution and the RAW resolution do not need to be the same, and this may also be used to reduce noise.

The above is purely a technical point; how strong this impact a real image i have no idea (i am an old timer DSP partially expert, just an amateur photograph :). I suppose that with old 8 bit cameras was a lot more visible than with current camera, where you a bigger dynamic pre-JPEG conversion. For example, the equivalent of ETTR in audio recording is to maximize the recording level, but with a 24 bit recorder you do not care much (unless you are doing a professional recording).

Maurizio

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