Originally Posted by neuroanatomist
It depends on how you look at it. If you are in, say, Av mode and you take two pictures in the same light, one at low iso and one at high iso, the high iso one will look noisier. But that's because the camera adjusts the shutter speed to let in less light at the higher iso. That is, the signal noise ratio is lower (ie, the picture looks noisier) at high ISO because the signal decreased, not because noise increased.
If, on the other hand, you take two pictures with the *same light*, that is, same lighting, same shutter speed, same f number, but with different ISO (eg, you set the camera to manual mode, take one picture, raise the iso, then take another picture), then the photon noise will be exactly the same for the two pictures. As you know, photon noise results from the fact that photons arrive on the sensor with a Poisson distribution. Expected standard deviation thus increases like the square root of the signal, so snr goes like 1 over the square root of the signal... ISO does not show up in the equation
Read noise- or shadow noise as you call it- is different. At higher ISO, read noise tends to be lower. I don't know anything at all about how cameras work, but I'm guessing this is due to the constant effect of electronics operating after the amplification of the signal done at high ISO... if you amplify then add noise, you have less noise than if you add noise then amplify.
The relevant comparison in the choice between cropping and using an
extender is the latter one. Whether you crop or use an extender, the same amount of light will strike your picture, so long as the aperture is the same. Thus using an extender actually lets you use a higher ISO with the same amount of photon noise, and so the total noise is actually less with the extender in some cases.
A while back, someone on this forum (probably Daniel) stated that this was a major factor for him in choosing extenders over cropping.
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