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From: jer@peora.UUCP (J. Eric Roskos)
Newsgroups: net.rec.photo
Subject: Re: film sharpness
Message-ID: <1166@peora.UUCP>
Date: Tue, 25-Jun-85 15:26:34 EDT
Article-I.D.: peora.1166
Posted: Tue Jun 25 15:26:34 1985
Date-Received: Thu, 27-Jun-85 05:21:17 EDT
References: <2807@decwrl.UUCP>
Organization: Perkin-Elmer SDC, Orlando, Fl.
Lines: 86

Um, while I will agree with a lot of what you said [I posted the original
article you are commenting on], a few of the statements aren't entirely
accurate (though close):*

>  1.  Reversal films such as Ektachrome are inherently finer grained
>  than the equivalent negative film, because in the negative film
>  the image is made up of the larger grains of silver which pick
>  up light better.   In a reversal film, the image is made up of what's
>  left after the large grains are removed in the reversal process, i.e.
>  the smaller grains.

Kodachrome film does not use silver, I don't think.  However, I believe
other reversal films do.  This is one reason why Kodachrome has better
grain/sharpness.  But, negative film does not leave behind the silver
grains either.  The silver grains produce "dye clouds" during development;
but during the last step of the development process (the "Blix" step in 2-
step processes), the silver grains are chelated out with EDTA, leaving only
the dye behind.

The main reason why negative film has a POTENTIAL for greater graininess
is that negative-working paper has higher contrast, and consequently can
make the graininess be exaggerated.  The reason it has higher contrast is
that print film has lower contrast than reversal film.  The reason for THAT
is that print film has a greater "exposure latitude" -- a wider range of
luminance produces different densities in the film than in reversal film,
because reversal film has the constraint that the densities produced in the
film are those used to actually view the image.  Along with the wider
range of exposure latitudes comes a smaller relative difference in densities
between two luminances (in the linear portion of the characteristic curve),
since there is only a certain density range that is usable, for practical
purposes.  This is a design decision in the manufacture of negative film.
Just as B&W negative film doesn't give the same densities in the
negative (e.g., there is a visibly-grey base+fog density, Dmin) that appear
in the print, so you can adjust the appearance of your print by varying
the exposure of the paper, so also color negatives do likewise.

> 2.  But market conditions are that most pictures are made with
> color negative film, therefore they put more effort into the negative
> films.  An example is the new range of high performance print films.

Actually, it's not "market conditions".  I wrote a letter to Kodak about
this a long time ago, asking "given the fact that you make negative film
specifically for making prints, why is it that people claim the quality of
reversal film (for prints) is so much better?  Why don't you optimize the
negative film for making prints, and the reversal film for making slides?"
Kodak wrote back (just recently) and said, in essence, "we do.  It's
just that some people prefer high-contrast prints and saturated colors.
We make the negative film to suit what are considered normal print properties
by professionals in the industry.  It's just a matter of what the
individual prefers."

> 3.  Kodachrome has an advantage over Ektachrome because the emulsion
> is thinner.  This is because the dye is added in during processing in
> the Kodachrome method.

This is true.  Despite my dogmatic Realism-based opposition to people's
obsession with Kodachrome, actually it is very good film, for slides.
However, print film has another interesting advantage that is not related
to emulsion thickness -- most of the print films have 2 emulsions in each
color, one faster (but coarser) than the other.  This is why many people
overexpose VPS film (which is rated at ASA 160) by exposing it at ASA 100;
it exposes the finer emulsion more, and gives a finer grain.  You can do
this with print film because of the above exposure latitude.

> But back to reality, aren't the full-page spreads in Time, Sports
> Illustrated, etc, good enough?  They're mostly taken with 35mm film, hand-
> held...

Well, except that they are usually printed through a halftone-screen, which
introduces a severe "grain" of its own! ... actually Audobon Magazine
experimented with printing without halftone screening about 8 years ago.
This produced very beautiful prints, although many of them had flaws where
the dye didn't go on uniformly.  But I haven't seen many magazines that do
that (and I really don't know how Audobon did it, either).

---------

*Of course, venerable and more experienced photographers in here may well
find errors in my statements, too!
-- 
Shyy-Anzr:  J. Eric Roskos
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