PanoTools:
Re: dynamic range and noise (yet again)
Simon Minshall 2005-Oct-27 18:29:06
Pedro,
Have you done the experiment?
My first reaction is that your shadow areas will be captured more
noise than signal at low exposure levels and noise+noise=noise you
may end up with brighter noise in the shadows. There's no magic at
work. If you've got low signal there's not much you can do about it.
THe other thing is that frame averaging will reduce random noise but
will increase fixed pattern noise. You may need to measure the FPN
and subtract it out.
My experience with frame averaging is that it is more effective with
large numbers of frames, i.e., 50-100 images.
Not to say you shouldn't try it, though. It may yield interesting
results.
Do you have the means to do it? Averaging in Photoshop is awkward.
imagej http://rsb.info.nih.gov/ij/ can do it
as can imageMagick
Simon
On 27-Oct-05, at 10:04 , pedro_silva58 wrote:
> 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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