Tom Ferguson (tomf2468@pipeline.com)
Tue, 09 Mar 1999 08:26:51 -0800
Liam Lawless <lawless@vignette.freeserve.co.uk> wrote:
>The other day Tom Ferguson raised an interesting point about
>reciprocity on APH, which could have possibilities. I have not
>noticed the phenomenon myself and I have not studied reciprocity,
>so I wonder if anyone could enlighten me as to whether reciprocity
>is purely a function of time, or also of intensity? What I'm
>thinking - obviously - is that IF reciprocity failure occurs in the
>shadows (where the intensity of the exposing light is greatest) before
>the highlights, it could be a useful contrast control... if it proved
>to be sufficiently controllable. <SNIP>
Kevin O'Brien <kob@paradise.net.nz> wrote:
>The current Kodak data gives exposure within 1/3 stop for
>1/000sec -100sec for various lith films without any change in contrast.
>These are graphics films and APH is probably similar as both short and
>long exposures are common in the industry.<SNIP>
Richard Lahrson <tripspud@hooked.net> wrote:
>Reciprociity, in general camera photography does effect the low values
>more than the highlights; that's why most manufacturers recommend
>decreasing film development to compensate. The effect on lith film
>is much less because of the steep slope of the D log E curve. <SNIP>
Reciprocity failure affects the areas of film receiving small amounts of
light. A certain amount of light energy is needed to get past the film's
threshold, this energy is "wasted" (not recorded). What happens in the
extreme is this: the areas receiving lots of light react "normal", the
areas receiving small amounts of light may or may not get past their
"threshold" and are thus under represented. As to Kevin's comments above,
because we are using this lith film in a soft working con tone developer, I
SUSPECT that the manufacturers time and reciprocity data is invalid.
My experiments with APH have been with Dave's LC-1 development method.
Using 2:2:6 for LC-1 developer (VERY soft developer) my density range (of
original neg, intended to fit into an enlarged positive ranging from 0.45
to 1.05) was 1.0 at 30 seconds, 1.1 at 1 minute and 1.2 at 2 minutes and
1.4 at 4 minutes. Using LC-1 at 2:1:7 (a more useful, general low contrast
developer for enlarged positives) my density ranges were 0.75 at 30
seconds, 0.85 at 1 minute, and 1.0 at 2 minutes. Oddly using LC-1 at 2:0:8
(normal contrast developer) I didn't see any difference in density range at
30 seconds or 1 minute?? I've only run these tests once.
I think I must have the worlds dimmest enlarger ;-(
I never realized this before. But using my (very old) coldlight for making
16x20 enlarged positives I am using 30 seconds as my "standard" time, and
varying the exposure via F-stops (in order to avoid the above reciprocity
failure confusion).
Hope this helps.
Tom
tomf2468@pipeline.com
http://www.thefstop.com/tf.html
This archive was generated by hypermail 2.0b3 on Sat Nov 06 1999 - 10:09:02