From: geoff@pmafire.inel.gov (Geoff Allen)
Newsgroups: rec.photo
Subject: Re: Pre-exposure to implement Zone System??
Date: 29 Sep 92 16:17:21 GMT
Article-I.D.: pmafire.1992Sep29.161721.29606
Organization: Roller Coaster fanatic with no hope for a cure

In article <1992Sep24.234031.17143@odin.corp.sgi.com>,
donl@glass.esd.sgi.com (donl mathis) writes concerning pre-exposure to
control contrast of transparency film:

>> This appears to me to be the most promising approach to handling
>> transparency contrast, and has the added benefit of improving the shape
>> of the curve.  I don't know of anyone who's worked it all out, though.
>> The problem boils down to the mechanics of consistent preflashing, and
>> hopefully on demand in the field once you know how much you need.
>> After that, a little testing, and you're done!
>> 
>> As to changing the color balance -- scary!  Good luck!

And schumann@msus1.msus.edu writes:
>Ansel Adams briefly discusses the possibility of using pre-exposure to 
>control contrast with color materials in his book _The Negative_. 

He also touches on the subject in _Polaroid_Land_Photography_.

>As I recall, he recommends a pre-exposure to Zone II.  But anyone who 
>is interested should go back and check _The Negative_ in the section 
>where he briefly covers the color Zone System (it begins around page 
>95) to be sure of his recommendations.

In the Polaroid book, he mentions II or III as a ballpark for where to
expose, depending on the film.  As always, experimental data would help
give a feel for it.  The danger is that as you pre-expose higher on the
scale, you will tend to get things to look more like bad fogging than
just a lightening of the shadows.

>If anyone tries this, I'm sure there are people on the net who would 
>be interested in the results.

I've only done thought experiments, and some plots of what happens to
the exposure when you pre-expose.  I did shoot some test shots once, but
they didn't turn out (basically, they looked badly overexposed, and over
the entire film surface (i.e.  including the part that's usually black
because it's covered by the film holder), so it wasn't the exposure. 
Maybe I messed up in handling the film.  Dunno.)  Haven't tried anything
since, because, well, taking test shots is *boring*.  I confess to being
more of an empiricist, sort of an anti-donl [ ;^) ], and I've been doing
most of my photography lately in the desert, and there aren't any dark
shadows in the desert, so I haven't had a chance to test it out.  I
mean, I even *tried* to deliberately pick a contrasty subject a while
back, just to shoot once without and once with pre-exposure.  So I
picked a scene at Massacre Rocks State Park here in Idaho with nice,
dark shadows, but, alas, with all that nice desert sun reflecting off
that light desert dirt, there were only 3 or 4 stops to the whole scene. 
So I still await practical results from real life. 

The difficulty, of course, is in making the pre-exposure.  Ansel used a
diffuser that he placed over the lens, but with that, I wonder about
adding a color cast.  My thought is a grey card exposed in the ``main''
light (e.g. sun).  This would cause no color problems in the lit areas
(same color, plus the exposure is a small part of the total in the
highlights), and help to counter the dreaded blue shadows in the shaded
areas, by making part of the exposure with less-blue light.  After
thinking of this potential, I wanted to see if Ansel said anything about
it, but he just hints at it and doesn't give many details.  Working it
out is left as an exercise for the reader. 

The challenge in using a grey card is exposing the blasted thing. 
Presumably, you've already set up and focused and all that, so you don't
want to move the camera, so you've got to hold the card in front of the
camera, but take a reading from the camera position, and make the
appropriate settings on the camera, and make the pre-exposure, all
without changing the position/angle, etc. of the grey card so you don't
change the exposure.  An assistant or clamp of some sort would prove
quite useful here.

If I ever get any results with it, I'll be sure to let everyone know.

Geoff, Mr. Empirical

-- 
Geoff Allen          \  Many people come, looking, looking, taking picture....
uunet!pmafire!geoff   \  No good.... Some people come, see. Good!
geoff@pmafire.inel.gov \   -- Nepalese Sherpa, quoted by Galen Rowell 
Newsgroups: rec.photo