[pure-silver] Simple Step Wedge Testing
- From: "Breukel, C. (HKG)" <C.Breukel@xxxxxxx>
- To: "Pure Silver (F) (E-mail)" <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 14 Mar 2005 13:31:31 +0100
Hi,
I recently came across a new (for me, that is..) and neat way to do 4*% =
film (zone) calibration at: =
http://www.apug.org/forums/article.php?a=3D104
Very brief : place a 21 steps Stouffer step wedge on top of your test =
film in a 4*5 holder, aim camera at a uniformly lit surface, measure =
(for Zone V), add 5 stops (so a Zone X exposure), and look if step 19 =
gives you a just above max black on your printing paper (or if you have =
a densitometer a density of 0.1 ). This will give your personal speed. =
Than look were Zone VIII lands for determining your development time.
At first I could not understand why step 19 should be the key point for =
the speed, but than I realised that you should actually (probably) =
consider the step wedge as a ND filter with 1/2 stop spacing (0.15 =
density differences). The nett density on my step 19 is 2.91, so step 19 =
can be considered as a 9 2/3 stop ND filter (9*0.3=3D 2.7 and 0.2 beeing =
2/3 stop). So if I let a Zone 10 exposure (10*0.3=3D3) hit the film, =
step 19 will "hold back" say 2.9 density, leaving 0.1 hitting the film, =
and that's your speed point. Like wise step 18 will be reproduced with a =
half stop more transmission (but not necessarily with half a stop (0.15) =
density, because this depends on development, if I understand this =
correctly).
So deviations from step 19 beeing 0.1 or first discernable black means =
adapting your first choosen film speed.
I think I understand, but if there are flaws or further remarks, don't =
hesitate to comment.
I envision that in theory you could do all your testing with just 1 =
exposure on 1 sheet of film. I have these 1/2 inch by 5 inch 21 Stouffer =
stepwedges. One could tape 4 of these on 1 sheet, expose, cut in 4 =
pieces after exposure, develop for 4 different times, and read the =
densities (I do have a densitometer). The only point is I do not exactly =
know waht to plot on the X axis: just the Stouffer step numbers, or 3.0 =
(Zone X) minus the exact densities of the Stouffer steps, or..?..
Best & thanks in advance,
Cor
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