Originally posted by KungPOW
If you do the wrong things, you can get lots of noise at any ISO setting.
If you do the right things, you can get ISO 1600 to look better then ISO 400 done wrong.
Worst case: Underexposed JPG, with in camera sharpening maxed, then pushed to correct exposure in post process.
Best case: slightly (0.5 to 1.5 stops) overexposed RAW file, confirm no channels are clipped with the histogram, pulled back in PP, then add sharpening etc.
Yes, this is more or less right.
Noise is in every picture you take at any ISO value. Actually the noise level at any ISO value generated by the sensor is the same.
Noise however is linear with the sensor temperature. The only good way to lower the
absolute noise level generated by the sensor is by cooling it.
The real question is: when can we see noise in our picture? Answer; when the signal to noise ratio is so low that the noise becomes noticable.
With higher ISO values you amplify all (analogue) signals coming from the sensor. It is the bad signal to noise ratio that makes you see noise then.
In black / dark areas of any picture the signal is very low, making the s/n ratio bad. Therefore in dark areas you are likely to encounter noise first.
So, under exposing a picture at ISO 100 and later cranking the picture up in post processing is almost the same as up the ISO value at picture taking (not quite, digital multiplication does not have the same resolution). It will make you see the noise all over the picture.
Using a high ISO value will amplify a weak sensor image signal together with the sensor noise signal, resulting in visible noise all the time.
I hope this helps you understand noise better.
- Bert