"bmoag" <aetoo DeleteThis @hotmail.com> wrote:
>Some published reports suggest a tolerance of no more than one tenth of an
>f-stop to overexposure.
It had better be at least that tight!
>Hence camera makers go to all kinds of lengths to increase the apparent
>dynamic range by fundamentally using underexposure, exposing for the
>highlights and amplifying mid and low range signals.
None of that will increase "apparent dynamic range". And saying
they both underexpose and expose for the highlights is a
misunderstanding of exposure technical details.
It is not something that *camera makers* do anyway. It is close
(but not quite right) for a description of the best technique
that a photographer can use to assure exposure that makes
maximum use of the dynamic range available from a digital
camera, whatever its dynamic range is.
Basically the technique is to set exposure so that the brightest
highlight _with_ _desired_ _detail_ is at maximum. That will do
two things. 1) clip on any highlight which is brighter, and 2)
provide the most detail possible in the shadows (i.e., it uses
the entire dynamic range available).
It works for both digital and film, but is excessively difficult
to accomplish with film and extremely easy with digital.
>Using raw you can control this process to some degree yourself.
Saving the raw data does allow for "amplifying mid and low range
signals", in order to compress the dynamic range of the camera
into whatever is available for display (paper, CRT, etc.). The
difference between film and digital is that with film you
destroy the raw data when the film is processed, and can never
do it again. With digital you can re-process the same raw data
in many different ways (just as if you re-exposed it on different
film or used different developer).
The effects obtained are the same, but the actual mechanisms and
the ease of accomplishing them are very different between film
and digital.
>However realize that while the eyeball is estimated to have an 11-18 fstop
>dynamic range this is really an illusion as well. Your eye shifts
[snipped]
I don't see any significance to discussion of how the eyeball
adapts to scene brilliance, since that is *not* normally
available via current display technology.
"Underexpose and expose for the highlights" is a generic error
in perception of exposure. It assumes an "average" scene will
be something close to 18% gray, and that the dynamic range of
the recording medium can record detail at all brightness levels
above that level. Neither are necessarily true, though it
commonly works out to be so when film is used with "generic
scenes". Hence film photographers have used it as a rule of
thumb that they rely on, and settle for "fudging" to adjust when
it is not true. (Instance when it is not true are snow scenes,
night photography, or scenes with light sources included, e.g.)
That works with film simply because film will greatly compress
the highlights, thus allowing significant adjustment in post
processing to get at least some detail (and often detail in
highlights is not significant and can be grossly distorted
without damaging the image).
The point is that with film and averaging, exposure can be
targeted at the middle of the recording medium's dynamic range,
and then either shadows or highlights can be "recovered" with
post processing when it turns out the average is not actually
18% gray. That is *often* necessary simply because averaging
incident light meters are used. The only way to avoid it is to
have well calibrated equipment and to use spot metering (which
is neither convenient nor quick).
A digital sensor does not compress the highlights, but it does
have more dynamic range. And there is the huge advantage of
immediate feedback to demonstrate the accuracy of the exposure.
The point at which highlights are clipped (or not) can easily be
set with one or two test exposures (which is much faster and far
more accurate than using spot metering). Hence exposure within
less than 1/3 an fstop is easily obtained, at *exactly* the
point that provides maximum dynamic range. It is *not*
"underexposure", but rather is extremely accurate *correct*
exposure.
The methods for analog an digital are somewhat different, but
the desired effect is often exactly the same. It is also true
that there are some significant differences in the actual
results. That is true because the way that film compresses
highlights can of course be used for effect (when accuracy of
detail is not significant), while digital's huge amount of
highlight detail has the opposite effect (for when great
accuracy in the highlights is useful). The shadows suffer
identical problems with noise, and digital has the advantage of
more dynamic range than film.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd DeleteThis @apaflo.com
>> Stay informed about: high dynamic range in P&S ?? 
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