WayneS wrote:
> At 05:53 AM 11/18/2008, Chuck wrote:
>
>> Are any of these high pixel count cameras able to deliver a quieter image by
>> combining pixels when used at lower resolution settings?
>>
>> For that matter, does down-sampling a noisy image in PhotoShop result in a
>> quieter image? I've never noticed but I don't have particularly noisy
>> cameras.
>>
>
> The theoretical noise reduction for random noise goes at the rate of
> 1/sqrt(N) where N is the number of samples. So, down sampling at a rate of
> 1/4 should cut the noise in half.
>
> The only way it can get better than this is if the noise is correlated in
> some way.
>
Wellll. The only way without using other techniques. And my experience
says that it depends on the size/nature of the noise and the individual
qualities of the downsampling app.
Some downsampling seems to reduce noise, while other seems to magnify it
so it is obvious even at smallish sizes. I can't say much about which
apps and which images sources are which. Some of my experience has been
with images posted by list members,where I often don't know what was
done. Surely you have noticed the images, perhaps particularly B&W,
where a 35mm frame reduced to ~800 pixels longwise has noise that looks
more like what one would expect at 100%.
It seems particularly important to me to control noise so it is minimal
at final web display size. The final size image usually requires some
sharpening for final use and that will tend to exaggerate any noise.
It's different, of course, where the grain/noise is an intentional part
of the image. Even there, control over the size and amount of the effect
may be useful.
For quite noisy originals, NR at full size, even if not completely
effective at that size, may result in almost noise free downsampled
images. So much depends on the detail density, or texture, or whatever
it is, of the image, that I don't know how to make more useful, detailed
generalizations.
In many subjects, noise/grain actually adds to the sense of sharpness in
the areas of fine detail, while it may become a distraction from the
smoothness of OOF areas and an objectionable mottling in deep shadows.
In those cases, I sometimes apply differential NR, high in areas with no
image detail info and low(er) in areas with lots of detail that would be
blurred. My NeatImage action in PS applies NR in a new layer, so masking
makes this pretty easy and quick to apply different amounts/parameters
to different areas.
Moose
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