Solarflare
No longer a newbie, moving up!
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Err .... !Okay, this has already been said but I feel its worth saying again. Noise increases in dark areas of an image due to stray photons that the human eye doesn't see.
No.
Actually, thats wrong.
There are two major variants of noise:
1. Dark current, i.e. noise because of electron movements from the temperature of the sensor, even if there is no signal at all on the sensor. This type of noise constantly happends. That means: long exposures will have more of it. You can get rid of this variant of noise by cooling the sensor, the more the better. Astronomers might use sensors cooled to a few degrees above absolute zero, reducing the dark current to neglible values. But since that is no option on conventional cameras, we use "long exposure noise reduction" instead. In that mode, the camera will take a second frame to estimate current noise levels (since this depends upon temperature, they wont stay the same).
2. Photon noise. This is noise because of the random distribution of photons. This matters more if you use high ISO, because ISOs above the base ISO of your sensor, you will amplify the signal from the sensor, i.e. get your output signal from less photons total. This kind of noise is why larger sensors with low base ISO can capute more color information than smaller ones with high base ISO.