From: donl@glass.esd.sgi.com (donl mathis)
Newsgroups: rec.photo
Subject: Re: Paper Exposure Times, etc.
Date: 7 Apr 92 06:02:36 GMT
Reply-To: donl@glass.esd.sgi.com (donl mathis)
Organization: Silicon Graphics, Inc., Mountain View, CA, USA


In article <72953@netnews.upenn.edu>, liang@saul.cis.upenn.edu (Chuck
Liang) writes:

|> >Well I hate to throw a wrench into  all this terribly impressive
|> >math, but I'd just like to add two words:
|> >                  RECIPROCITY FAILURE
|> >In other words, (to get back to the original question of whether
|> >ther's a way to calculate exposure times for BIG enlargements),
|> >all the math may get you close, but there's no way around doing
|> >test strips (I think).
|> 
|>   Instead of throwing a wrench, why don't you throw some light into
|> the matter and tell us something about the reciprocity failure
|> characteristics of the most popular paper types.  I've heard a lot
|> concerning reciprocity failure of films but nothing concerning papers.
|> I mostly use Kodak Polycontrast III and Kodak Ektacolor Plus papers.  

Kodak might have this information available.  I know they provide it
for the color papers -- it's in the data sheets.  I'm sure if you call
their information center, they can help you.  I've seen useful curves 
on Cibachrome, too -- in the "Complete Guide" from HP Books.  In color
papers, of course, the reciprocity failure is different in the three
layers, so you have the balance to worry about in addition to overall
exposure.

In a word, though, reciprocity failure in paper is a much more
significant factor than it is for the vast majority of film exposures.

|>   I don't think test strips are a better solution, unless you do all
|> your enlargements at just a few magnification settings.  I love
|> printing because it allows me to select out a part of the negative to
|> enlarge, so I can't afford to make a test strip for almost every
|> print.

I lost you, here.  I don't understand how selecting part of the
negative prohibits making a test strip.  I can't imagine printing
*without* test strips, under any circumstances.  Paper seems to be
subject to a wider range of variations than film, and in some cases,
might sit around for a long time getting slower and slower by the
month.  Given the variations in negatives, it seems like the only way
to find the *right* exposure.  I know there are mechanical techniques
for finding the approximately-right exposure, but if you're going to
bother to make a print, you might was well do it right, eh?  For color
prints, I only make "mechanical" prints when I'm not terribly concerned
about the color.  I just use the analyzer, which is usually close enough
for most people's sensibilities.  For a real print, though, it gets the
full color test strip (i.e. "ring around") treatment, because there's
no better way to get the color right.

You have to *see* a print to make a high-quality judgement about
exposure and/or color, and a test strip is the most efficient way to do
that.

- donl mathis at Silicon Graphics Computer Systems, Mountain View, CA
donl@sgi.com
From: tonya@hpldsla.sid.hp.com (Tony Arnerich)
Date: Fri, 3 Apr 1992 22:37:01 GMT
Subject: Re: Rectilinear Fisheye Lens & Panoramas
Organization: HP Scientific Instruments Division - Palo Alto, CA