I just wanted to point out, that there really is no difference in the effect of low light on the sensor - be it through higher ISO (and thus shorter shutter speed and/or smaller aperture) or through dark parts in your picture.
It's not that the camera cranks up the ISO/amplification in the dark parts, but the s/n-ration of the photo-cells in the dark parts is nonetheless worse than in the bright parts and you can definitely see more speckles and freckles in the dark parts.
That we don't see them so prominently just has to do with our eye (and our senses) beeing drawn to the brighter parts.
And yes, you're right: Sometimes the noise even becomes visible in medium-bright areas! This tends to be the case when these areas have little structure (like some out-of-focus background). In this cases the noise stucks out
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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