Digital BW, The Print

You've got it!  That's exactly the problem, and that's why you can never
simulate the response of one film or sensor using data from another film or
sensor.  You can't get a Tri-X look from a digital RGB sensor, and you can't
get a Velvia look from a Provia scan, and so on.

----- Original Message -----
From: "Jon Dubovsky" <entropy@...>
To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Wednesday, May 28, 2003 04:27
Subject: Re: [Digital BW] Digital, film, scanning comparisons


> > I agree with Austin.   Tell us precisely what information is
>  > missing.  Don't speak vaguely about the "curves" because we know what
>  > the response curve is of the color dyes used in the film.   So we
>  > know exactly how much to compensate the density by for any color.
>  > Or put another way:  because we know the shape of the film's response
>  > curves there is only one unique point on the spectrum that will
>  > produce a given density in all three dyes.  So what's missing?
>  >
>  > You say that other people here understand you, but I don't see them
>  > jumping in to clarify what you're saying.
>
> I think I need to weigh in again.  Perhaps no one read my previous post.
> *I* understand Anthony, at least on this point:
>
> It is a basic mathematical fact that the process of binning is *not*
> reversible under almost all circumstances.
>
> Let's say that we have a spectral input of amplitude A(F), where F is the
> frequency.  Let's take two simple response functions (the same applies to
> three, but I the extra lines clutter things up), r(F) and g(F).
>
> If we take two binning functions to collect r and g...
> R = integral over all F of (A(F)*r(F))
> G = integral over all F of (A(F)*g(F))
>
> Look at those last two equations.  If I give you the result of the
> integral (the R value, for instance) and the r(F) response function of the
> filter/CCD combination, can you tell me what the input A(F) is?
>
> Answer: for all but the most trivial A(F) and r(F), no.  For an arbitrary
> A(F), never.
>
> More simply:  if you know the exact spectral response of, say, the red
> element of a CCD sensor, and I tell you I got a red value of 130, can you
> tell me the original spectral input?
>
> Answer: no.
>
> --
> Jon Dubovsky ( entropy@... )
>
>
>
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