Digital camera noise is related to exposure by the square root of the exposure value (EV) difference (somewhat of a simplification as there are several types of noise).
- Adding one stop of exposure cuts noise by a factor of √2 = 1.4.
- Adding two stops of exposure (4X) cuts digital noise by a factor of √4 = 2.0.
The amount of actual image data relative to noise (random junk) is called signal-to-noise. Thus, two stops more exposure doubles the S/N ratio.
Where noise appears
High-key areas already have a high signal to noise ratio and thus show relatively low noise. Mid-tone and darker areas are most at issue. But even relatively bright subject matter like clear blue sky can shown quite strong noise under some conditions (especially with a polarizer, which makes it a very dark blue).
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