PanoTools:
RE: dynamic range and noise (yet again)
Paul D. DeRocco 2005-Oct-27 21:29:46
> From: pedro_silva58
>
> imagine we expose for the highlights, ie, measure exposure so as to
> take the highest important highlight as close to clipping as possible
> without actually clipping. we take, say 8 shots with that exposure
> value (with tripod, remote, etc). if the scene dynamic range is high,
> shadows will be severely underexposed, and mids somewhat underexposed,
> too.
>
> now, we open all 8 images, and add (photoshop-screen?) them together
> -- call it image S. this will increase the exposure the equivalent of
> 3 stops, lightening the dark parts (opening up the shadows). next, we
> do the same with 4 images only -- in fact, do it twice, once with the
> first 4 images, then with the next 4, then average them together, and
> call the average M. then, we also average all 8 images, and call that
> average H.
>
> image S should have an okay exposure in the shadows, possibly with
> less noise than a single long exposure. image M should be okay for
> the midtones, with less noise than a single longish exposure. and
> image H will have the same exposure as the original shots, but much
> lower noise.
>
> finally we composite the three images S, M and H, pretty much as if
> they had been bracketed. S will contribute the shadows, M the
> midtones, H the highlights.
>
> this is a lot more work than a simple 2 or 3 bracket sequence. what
> do we gain?
> - long exposures are prone to noise. adding several shorter exposures
> helps (this sort of thing is often used in astrophoto for the same
> purpose).
> - with bracketing, we have a single (somewhat noisy) highlight
> exposure. this way, we can considerably reduce noise in highlights too.
> - the less noise we have, the more we can sharpen, etc.
> - with the extra work, comes extra flexibility.
> - all cameras are limited in bracketing range. this way, the sky
> (your card memory?) is the limit.
> - it is possible to combine the ideas of range expansion and noise
> reduction (eg, take 8, average in pairs, then add the 4 averages: this
> would increase exposure 2 stops, and still reduce noise more than a
> single exposure for double the time).
Mathematically, the sum of a bunch of short exposures has the same amount of
noise as one longer exposure. The only reason I can think of for using a
large number of equal short exposures, averaged as you suggest, as opposed
to one short, one medium and one long, is to avoid certain sensor artifacts
that might occur as a result of clipping highlights. But I think modern
sensors have that problem licked. I've never noticed it in my 10D.
I think that if that provided a measurable benefit, then cameras would
already be designed to do that automatically for long exposures. All these
tricks are certainly well understood by the designers of sensors and digital
cameras.
--
Ciao, Paul D. DeRocco
Paul mailto:#removed#
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