Sunset image issue

Started 2 months ago | Discussions thread
Quarkcharmed
Quarkcharmed Senior Member • Posts: 1,523
Re: Exposure compensation and Highlight weighted metering

Kaspah wrote:

Quarkcharmed wrote:

It's debatable if ISO 'creates' noise but it definitely 'reveals' it.

I don't accept that ISO by itself creates noise because if you have a constant scene lighting, aperture and shutter speed the shot noise will be the same whether ISO is set to 100, 200 or even 400.

First I didn't say ISO 'by itself' creates the noise. I said it 'reveals' the noise. Also it limits the exposure(s) you can use without blowing the highlights.

The shot noise will be the same because the exposure - amount of light that fell on the sensor per unit area - will have been the same at each ISO setting.

Shot noise in the physical light hitting the sensor isn't exactly the same as 'visible' noise. The ISO setting affects what part of the signal will be eventually transformed into the 'visible' noise after all analog and digital transformations.

So in this case, raising ISO will make the image lighter without changing the exposure, and quite possibly even without clipping highlights, and so if the lightness of ISO 400 is not dialled down in post to what the photographer saw then of course the noise that is present will be more visible but raising the ISO did not create additional noise.

And as I am sure you are aware, in many cameras raising ISO actually reduces noise.

It's a bit misleading statement - yes I'm aware it's popular on DPR.

When you raise ISO, you limit the max exposure you can use without blowing the highlights out, and because of the reduced exposure you actually get more noise.

Raising ISO by 1 stop (e.g. 100->200) may reduce visible noise compared to a shot taken at ISO 100 and then with 'exposure' lifted by +1ev in postprocessing. That's what they mean by saying "raising ISO actually reduces noise". Technically it doesn't reduce noise, it reduces SNR in raw data, compared to lower ISO setting.

In practice, higher ISO means lower exposure and higher visible noise.

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Canon EOS R5 Canon EF 70-200mm F2.8L IS II USM Canon EF 24-70mm F2.8L II USM Canon EF 16-35mm F4L IS USM
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