From: acl on
On Mar 27, 2:39 am, John Sheehy <J...(a)no.komm> wrote:
> "acl" <achilleaslazari...(a)yahoo.co.uk> wrote innews:1174792863.245906.287730(a)l75g2000hse.googlegroups.com:
>
> > On the one hand, I find it hard to believe it's the compression, the
> > gaps between the levels that are present are smaller than the
> > theoretical photon noise,
>
> That still posterizes the noise and signal a little bit. You're not
> likely to see it with any normal tonal curve; you really need to increase
> the contrast quite a bit, and you will see it. For example, I remember
> shooting in extreme fog a couple of years ago, where I used +2 EC with my
> 20D, at ISO 400, and raised the effective blackpoint such that the dark
> parts of the Robins approached black. It brought up a bit of noise that
> would not normally be seen, with any exposure compensation level, while
> black was still anchored at black. Same with taking pictures of things
> reflected in glass over a white background, if you try to restore black
> in the processing.

Well yes, that is what I was thinking too (ie that posterising the
noise could cause problems under extreme adjustments), but didn't
actually see anything the couple of times I tried (by shooting in forg
and moving the black and white points). I also played a bit with
compressed and uncompressed raw files but could not see anything so
far. Maybe I was not extreme enough.

>
> > so basically the extra tonal resolution of
> > uncompressed raw just records noise more accurately [and since you
> > can't really see shot noise in reasonably high-key areas, that tells
> > you it's irrelevant resolution anyway]. On the other hand, who knows?
> > Maybe there is some indirect effect.
>
> Recording noise better is a good thing, and the same conditions record
> signal better as well (and allows the brain and algorithms to separate
> them better, as well).
>
> In this particular case, it is only likely to be seen in extreme
> blackpointing, or perhaps extreme sharpening.

Yes, obviously you'd expect to see a difference under conditions that
exaggerate small differences, ie extreme tonal stretching or
sharpening (which is local tonal manipulation, after all). But I
didn't.

Well I'll try to play with Paul's example and see what happens
(unfortunately I just remembered I have an early plane to catch
tomorrow so it'll have to wait a bit).