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[filmscanners] Re: Scene brightness and CCDs



Todd writes:

> Seems to me a good "S" curve algorithm could
> juice the midtones and still keep the highlights
> and shadows from from blowing and blocking, no?

Absolutely, and an S curve is what I had in mind (although I don't actually
know for sure what sort of adjustment the cameras are performing).  In order
to give images more "pop," the processing would logically increase the
change in luminosity in the midtones, where most of the information in a
properly-exposed image would be, and sacrifice detail in the highlights and
shadows.  A plain straight-line change would work okay, but then highlights
and shadows would be abruptly cut off, and they would contain more detail
than necessary for most images up to the cutoff points, to the detriment of
midtones.

In other words, I'd expect image-processing software to emulate the same
type of curve that is built into film emulsions, with a steep midsection and
a "toe" and "shoulder" at the extremes.  Of course, same curve = same
drawbacks, so highlights and shadows will suffer.  But unless you have a
magic display device that can actually display a very wide gamut and a very
broad dynamic range, you have to compress something.

Some digicams may be overdoing it, by forcing images into the
lowest-common-denominator gamut, which would be something like sRGB.  I
guess this is understandable for the consumer cameras, but I should hope
that the pro cameras are using something wider, or even allowing the user to
select a space for rendering of an image (including raw data from the CCD,
preferably, which would mean no manipulation at all).

Even better would be to do this all in the analog realm, with the raw output
signals from the CCD, rather than trying to adjust after conversion to
digital data, because some resolution will be lost in the conversion, unless
it is extremely precise internally (18-20 bits, for example).  But that
would be expensive, inflexible, and prone to misadjustment and environmental
influences, so I doubt that it is being done.

> Will the RAW captures from these cameras hold
> detail at the extremes under conditions of large
> brightness ranges? How many stops of brightness
> for the RAW captures?

I don't know.  It depends what "RAW" means for a given model of camera.

Also, even the pro cameras today are using CCDs that are too small to
provide the full benefits of CCD capture.  The smaller the pixel, the less
charge you can hold on a given photosite, and the smaller the dynamic range
of the CCD.  You need big pixels to provide lots of range, and to maintain
resolution, then, you also need a big CCD.  This is an argument in favor of
full-frame 24x36 CCDs, but nobody has them yet (practically).

Another problem is thermal noise, but you'd have to actively refrigerate a
CCD to really drive it down and realize the full benefits of CCD range at
the shadow end.  That's probably not practical in a portable camera.
Fortunately, it shouldn't be much of an issue except for really long
exposures or really warm environments.



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