Just concentrate on one thing for this post
So I wouldn't say 'high ISO reduces noise', what I'd say is that 'high ISO reduces visible read noise'. However, this language doesn't really help people that much, and gets dismissed as technobabble. I think when I reduce the subject to its basics, it's pretty clear.
i think people just don't know what that means. i don't know what that means, even though you've clearly explained it here at least one, and what the effective difference actually is.
For the photographer, there is no difference. For some who know a bit of electronics there is a difference.
For the photographer, what is usually meant by 'noise' is the speckly stuff in the picture they look at. For them the statement is true without further qualification++. However, if someone with a little electronics knowledge+++ looks at the statement, they object, because they understand that high ISO involves more amplification, and that amplification will increase the amplitude of the noise measured in volts, or measured in analog to digital units, which is true. What's going to happen in processing, though, is that the amplified signal gets mapped back to a normalised range of output tonality, which has the effect of reducing the visible noise in the final output down below where it would have been without the ISO adjustment. Therefore I need to qualify my statement with 'visible'. I've chosen that in the hope that photographers might still understand it while it still satisfies the technical quibble.
Now, the next point is to do with the type of noise we're talking about. There are two sources of noise by and large. The photon shot noise, which is caused by the random arrival of photons at the sensor, and the read noise, which is the electronic noise generated within the camera. Photographically speaking, the shot noise is generally the noise in bright and mid tone areas of the image, shot noise is the shadow noise. The amount of the shot noise in the image++++ depends absolutely on the exposure, so as the exposure reduces the visible shot noise increases. Generally, though the read noise decreases as ISO setting increases, and this is what I'm talking about, so I make sure I refer to 'read noise' and not just noise.
++ although it still confuses those who believe that there is an unbreakable link between ISO setting and selected exposure. To make it absolutely clear for these people one should say also
for the same exposure , although the risk then is then that many, like chuxter, misunderstand exposure to include the ISO setting as well incident light, aperture and shutter speed. My good friend Joe James talks about 'exposure' (excluding ISO) and 'apparent exposure' (including ISO) to distinguish them, but I'm not so sure that makes it any less confusing for those who don't know what exposure means, and it just irritates those that do.
+++ the problem here is those with a
little knowledge. Those with a lot tend to understand from the context that one is talking about the photographer's perspective.
++++ and even here I need to make a qualification, and say 'so far as the photographer is concerned', because the
amount of shot noise goes
up as exposure increases, but the noise to signal ratio goes
down , and in terms of visible noise in the image, it's the noise to signal ratio that you see.
--
thomas
I knew you'd think that. You are not a guy who makes generalizations easily.
