Darkroom basics

"J. Christopher" <JCFrey.RemoveThis@gmail.com> wrote
 > Nicholas O. Lindan wrote:
  > > Polycontrast filters
 > I’m using Ilford filters with Kodak paper and have been having
 > problem with contrast.

What sort of problems?

FWIW, I normally use Kodak filters with Kodak, Agfa and Ilford
paper. I haven't noticed any difference when changing to
Ilford filters.

Variable Contrast (VC) papers are not really as advertised,
so don't be surprised if #1 filter shows less contrast
than #1/2 and other peculiarities.

It is instructive to plot the curves for VC paper. Either
step tablet and densitometer or test strips and a grey scale/
eyeballs works.

You will find:

o The grades are not evenly spaced
o Sometimes they run backwards
o Sometimes they are straight from toe to shoulder
o Sometimes they curve up " " "
o Sometimes they curve down " " "
o Often they have humps and bumps
o There is nothing constant (or logical) about paper speed
when changing filters except that the very high grades are slower

* * *

For Ilford MG-IV/RC & Kodak filters I found:

Rationalization of paper grades, ~.6 stop/grade

Filter -1 0 1.5 2 3 4 5
Grade 0.0 1.5 2.0 3.0 3.5 4.0 5.0

Exposure required for given density

1.90 OD 5.4 4.4 3.9 3.7 3.5 3.9 3.9 Stops
.13 OD 0.5 0.7 0.7 0.8 0.9 2.0 2.5
Delta 4.9 3.7 3.2 2.9 2.5 1.9 1.4

Exposure correction to hold the same highlight or shadow

Shadow 0.0 -1.0 -1.5 -1.7 -2.0 -1.5 -1.6
Highlight 0.0 0.2 0.1 0.3 0.4 1.5 1.9

When changing grades take the value for the new grade
and subtract the value for the old grade to compute
how much to change the exposure.

If the shadows are the important part of the pic then
first find the best shadow exposure and then change grades,
using the shadow correction values above, until the
highlights are at their least objectionable.

If the highlights are the important part then first find
the best highlight exposure and then change grades, using
the highlight correction values above, until the
shadows look about right.

* * *

Example 1: to move from filter 2 to 3, keeping the same
highlights but making the shadows deeper:

Increase exposure by 0.1 stops (0.4 - 0.3)


Example 2: to move from filter 3 to 4, keeping the same
shadows but making the highlights lighter:

Decrease exposure by 0.3 stops (-2.0 - -1.7)


Example 3: to move from filter 2 to 1.5, keeping the same
shadows but making the highlights darker:

Increase exposure by 0.2 stops (-1.5 - -1.7)

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.
To reply, remove spaces: n o lindan at ix . netcom . com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/<!-- ~MESSAGE_AFTER~ -->