PanoTools:
dynamic range and noise (yet again)
pedro_silva58 2005-Oct-27 18:04:49
greetings, all!
here's an idea, related to dynamic range and noise. like bracketing,
it would only work for static subjects.
if this makes any sense, it shouldn't be too hard to implement in,
say, photoshop, or a small dedicated program.
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).
so... does any of this make any sense?
cheers,
pedro
p.s. it is often said that cameras are limited to 3 bracketed shots.
that's certainly true of any cameras i've owned, but not of all
cameras. eg, the 1ds2 does up to 7 frames, the d2x and d2h up to 9.
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