by Mike McNamee Published 01/12/2013
Noise and Bit-depth
Noise is worse in shadow areas because the strength of the signal is low and forms a larger proportion of the total signal.
In the lower set of graduated scales, the full exposure creates an even set of patches across the range and below that an under-exposed image is piled up to the left, ie the image is compressed into the shadow region.
If this is then expanded (by adjustment in Photoshop) the bands are spread out more widely across the full tone range. This is reducing the bit-depth, the effect of which is shown in the bottom strip of images in which the banding becomes very obvious.
The noise level was measured across the Macbeth Chart for each of the bracketed exposure images. It falls quite rapidly then levels out but there is no noise penalty for an over-exposed image. By the same token there is no significant noise penalty in any of the exposures close to the optimum (mid-tone exposure).
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