Started Dec 20, 2013 | Questions thread
(unknown member) Senior Member • Posts: 1,321
Re: Why ETTR?

You are missing a bunch of things.

What we want to do is not minimize the noise. Instead we want to use parameters that maximize the signal.. which is the exposure.

Exposure is the light energy recorded by the sensor when the shutter is open.

Increasing ISO does not increase exposure. Only aperture and shutter speed affect exposure.

The sensor operates at one sensitivity level (usually called base ISO).

Above base ISO the analog signal level is increased by amplification after the shutter closes and before the signal reaches the analog to digital converter. The signal and noise are amplified equally. The amplification makes the image appear brighter. It does not increase the exposure.

The electronic noise added by ISO amplification is less than the sensor's inherent read noise, so using ISOs above base ISO does not add noise. What happens is the noise remains constant but the signal level is lower because the meter tells you it is possible to use a shorter shutter speed or narrower aperture. Then the camera amplifies the signal to make the brightness match the meter's prediction.

Think about maximizing exposure in order to get the most from your sensor. Due to underexposure the signal-to-noise ratio and dynamic range decrease as ISO is increased above base ISO.

ETTR can maximize exposure when you use the lowest ISO possible that is consistent with the goal of the photograph you envison (freezing motion and, or the required depth of field).

PS There are more details and different cameras operate differently. I intentionally omitted these topics. These omissions do not change the mantra "maximize exposure of the sensor".

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