ISO 100, OMD-EM1II and firmware 3.0?

Started 2 months ago | Questions thread
bobn2 Forum Pro • Posts: 61,718
Re: Meaning of base vs. extended ISO

Chris Noble wrote:

bobn2 wrote:

katastrofa wrote:

My photos look more noisy when I increase the ISO. Why?

Changing the ISO sets the meter so that you choose to use a lower exposure. Lower exposure means less light energy. Less light energy means fewer photons captured in the image. The signal to noise ratio in the image goes as the square root of the number of photons, so if you capture fewer, you get a lower SNR, which you say is 'more noisy'.

You are mixing changes in exposure vs. changes in ISO setting.

With respect, I'm mixing nothing.

Katastrofa's observation is correct even if the exposure does not change (using manual metering for example). The ISO gain introduces additional noise on top of the exposure noise.

That is simply false. It's provably false with suitable examples, which I've posted here several times, but don't have readily available here (if this conversation carries on, I'll post them again, when I can). You've bought into the false narrative that 'ISO' is 'gain', 'ISO noise' is 'amplification noise'. Let's work it through. The component of noise that is electronic noise is called 'read noise'. It come from the two ends of the read chain, in the main. The first component is the noise of the source follower amplifier which reads the charge from the pixel. This is actually a current noise, not a voltage noise, the voltages from the pixel are actually usably large without additional voltage gain (up to the order of a volt or more full scale in a CMOS sensor - enough that the variable gain amplifiers will generally have a base gain of unity). The second component is the noise from the ADC, and the problem here is that the operational bandwidth of the ADC is large, because millions of pixels need to be digitised in a short time, so the conversion becomes noisy (this is different from quantisation noise). The change in voltage gain is simply to boost small signals, when signals are small, typically in low light, up above the noise floor of the ADC. The SNR of a good integrated voltage amplifier might be 100dB or more. When we're dealing with SNRs of 40dB or so, as we are in digital photography, that's irrelevant. It was this failure of the standard ISO narrative that got me started on working out how ISO really worked. It was very clear that for the amplification noise theory commonly propounded on websites, and which you've put forward here, to be true, digital camera designers would have had to be truly incompetent. I've worked with Canon in the past, and I know they are thoroughly competent, so it just didn't. I did my research, read the relevant ISO standards, learned from the likes of Iliah Borg, and I know that the 'ISO is gain' and 'ISO noise is amplification noise' story is complete claptrap.

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263, look deader.

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