Why expose ETTR and how to do it?

Started 4 months ago | Discussions thread
Erik Kaffehr
OP Erik Kaffehr Veteran Member • Posts: 5,200
Re: Why expose ETTR and how to do it?

strawbale wrote:

Erik Kaffehr wrote:

SrMi wrote:

Erik Kaffehr wrote:


Nice summary, thanks.

There is no big reason to overdo ETTR. Most cameras have pretty good SNR at base ISO.

What does "good SNR at base ISO" mean?

I assume that you meant that most cameras have at base ISO large enough DR (max. signal to noise "distance") so that losing a bit of DR due to underexposure is not a biggie.

No, I mean that SNR is good enough not cause significant noise in mid tones, like blue sky or facial tones. SNR is proportional to exposure, so if exposure is reduced SNR will also reduce.

This figure from DxO-mark illustrates this, they peg 'good SNR' at 38 dB. All three cameras are above 38 dB at base ISO. Panasonic crosses the line at 117 ISO, Sony A7rIV at around 500 ISO and Hasselblad X1D at 800 ISO. Note that dual gain does not affect SNR. That is because it does not change the photon count, just the voltage swing that the photon count is resulting in allowing better readout.

On a camera with a Sony sensor, it may not be a great idea to increase ISO get more ETTR exposure. ETTR is about exposure, not ISO.

Dual gain sensors have kind of two base ISOs. Jim Kasson wrote how to optimize for that by increasing ISO in discrete steps.

Dual gain improves DR, mostly. DR is maximum signal, divided by readout noise. What dual gain does is to reduce the capacitance of the photodiode, so it reduces maximum signal, but when reducing exposure, the full capacitance would not be utilized anyway. But reducing the capacitance increases the voltage swing and that allows a cleaner readout.

Here the effect of dual gain is clearly visible.

Best regards


Beginner's question: "how much better" is a SNR (18%) of, say, 42 compared to 36 (at same ISO)? Is 3dB difference halving the noise?

I would think that 6dB would be halving the noise.

Just to say, this is a very good paper related to moise:


Best regards


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Erik Kaffehr
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