The Joy of Pixel Density

Started Jul 13, 2008 | Discussions thread
aclo Regular Member • Posts: 480
Re: Thanks John

Brad Nichol wrote:

On the subject of noise a method I often use with compacts under low
light is to take 4 or more identical exposures and blend them in
registration at 50% in photoshop, what is really obvious by this
method is that once you have about 4 images blended together the
shadow noise has pretty much been all cancelled out leaving just the
real picture information, so even with really high MP counts we could
get totally clean results if we needed it for some non moving
subjects.

Averaging four frames reduces noise for exactly the same reason averaging 2x2 pixels (ie downsampling) does: because the noise across the four frames or four pixels is uncorrelated (almost).

The fact that the demosaicing process causes this noise to not be completely uncorrelated does seem to make downsampling less effective than it could (low frequency noise isn't removed very well), but overall it works.

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ck3
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