Re: [Leica] Cold light, dichroic (longwinded reply and editorial!)

["Dan Post" <dpost@triad.rr.com>]

Let me make a few comments based on some of my experience with several types
of enlargers, ranging from my first Federal, through the Durst, Beseler,
several Omegas, and now a Leitz V35. If you took the same lens, and the same
negative- using these on each of the negatives, and reading the relative
density of the same portions of the negative, you would find that because of
different designs, interior reflections, different degrees of the Callier
Efeect, and light sources, that all would render a different 'contrast' for
the same negative!

I always thought it was a good idea to use an enlarging meter, and though I
use a Beseler Color Analyzer (Boy- they are cheap nowadays!), it can
function as a de facto densitometer very nicely!

I make some target negatives when I test film- I shoot a 16x20 foam
core/matte board to which I have applied 8x10 targets- a neutral grey gard,
a white matte board and a black matte board- all of which were measured with
a reflection densitometer to find their true 'density'. When testing film,
it is a simple matter to read the corresponding spots on the negative with
the meter, and comparing them with the base plus fog to get the relative
densites of the images of the test targets. From this I can work out the
best development times for the enlarger and camera system I am using for any
particular film. Measureing the image of the white card and the black, I can
determine the pratical density range that the film will give, and thus the
proper grade of paper to use.
A hard and fast development time for your film is not possible; it should be
adjusted to your personal working habits, and done so that with YOUR
enlarger, and YOUR light source, you can get the results YOU want.
For down and dity purposes, I use a dichroic head in the V35; The dichroic
filters are coated with a metallic film of a certain thickness that passes
one color of light and reflects the complement- thus a yellow dichroic
filter will pass yellow light through, and reflect blue light from the
incident surface. The film is vapor deposited on a substrate, commonly a
soda-lime float glass. They are very resistant to fading as opposed to dye
based filters. In the color heads of all the enlargers and printers I've
seen, the filters are moved into and out of the light path with cams- each
filter impinging more of less in the light path to give the characteristic
color to the light. The V35 has a single dichroic strip that varies from a
yellow band pass on one end to a magenta color on the other, so it is
possible to also get variations in the filtration between grade which is
very useful as I will explain in a minute!
Since various heads are not normally 'calibrated' to any standard, as I have
found, I usually calibrate my film and enlarging system by using what is
known as a step-tablet.
It is a strip of film that has various densities- in my case, I use one
graduated in steps of increasing density in steps of .15- and ideally
represents a negative where the gamma is about .5. That is if you think of
each 'zone' or stop (log .30) of scenic brightness is represented by step of
log density .15, the staep tablet can be seen as representing a 'perfect'
negative, with all possible tones represented.
I make test strips by taking a sheet of paper, cutting it into 5 strips, 2x8
inches, and make a series of exposures- essentially contact printing the
step tablet on each strip in turn, and for each, changing the filtration
setting to each 'grade'- then I develop them, fix, was, and dry them, and
sitting down under a good light, I count the steps visible on each strip.
With the grade 5 filter- you may only get 3, 4 or five steps, while with the
00 filter, you might get eleven steps from a absolute white, to the most
black the paper is able to produce.
Since each step on the test strip represents a density change in a negative
of .15, then if your grade 5 filter shows 3 distinct steps, the density
range of that grade is 3x.5 or .45!
Likewise, if the 00 filter setting gives you a range of 11 steps, then your
enlarger can print a negative with a density range of- 11 x.15 or 1.65!
When I get ready to print a negative, I will read a spot in the negative
image projected on the enlarger base that looks like a shadow detail area
with the probe of the meter. I then 'zero' the meter to the right hand
scale, and then read the most dense area that would give me some highlight
detail- like the folds of a white shirt, for instance. The needle will
deflect to the right, in my case, and show a relative density, of - let's
say 1.05.
I then know that the approximate density range of that negative is 1.05, and
represents the same ramge as found on the step tablet strip that has as
nearly 7 distinct steps as I can determine- if you have kept a record of the
number of steps each grade of filter will produce, you can know immediately
filter to use to start your printing.
I found that this eliminates a lot of wasted time, and I actually use the
same technique to determine which time/aperture  is used to produce the
desired print density from any negative density, but that's another story,
and one I need to put on my website!
I have found that once you 'calibrate' your particular enlarger and lens
combination, for that particular paper, then it saves a lot of time and
wasted material. The new papers of today are remarkably consistent, and
unless you change brand or type of paper, then the particular setup you go
through is something that doesn't need to be done each time you go into the
darkroom.
I keep programs for each type and brand I use and so it is much easier to
get a lot of work done, with a minimum of wasted materials and time. Since
everyones printing system is going to be slightly different- the procedure
must be done on each enlarger you use. You would be surprised by taking the
step tablet and printing with a nr.2 filter on several different enlargers,
and lens combinations at the variation- some slight, some larger, in the
number of steps that show up on the test strip!
I've shown the system to several people and they have found that printing
can be fun, and that the long hours of tedium, getting that print just, just
right, are much shorter and more relaxing!
Cheers and best of light to you!
Dan (Long winded as always!) Post
of density that differs from the next by .15
- ----- Original Message -----
From: Mark Rabiner <mark@rabiner.cncoffice.com>
To: <leica-users@mejac.palo-alto.ca.us>
Sent: Saturday, May 20, 2000 6:57 PM
Subject: Re: [Leica] Cold light, dichroic


> Walter Levy wrote:
> >
> > Jeff Segawa asked how does a cold light head differ from a dichroic
diffused
> > head.  I'll tell you... I don't know.  All I know is I have to put these
> > acetate multicontrast filters between the cold light head and the
negative
> > stage.  With dichroic head, you can just dial in your desired
contrast...
> > handy!
>
> It's a graduated filter that passes in front of a blazing hot Hologon
bulb.
> As you turn the dial the filter is like a wheel which moves to a denser or
less
> dense part of its gradation
> If you use the same "number" of gradation  (Say number 100) often that
part will
> fade and you will have "bumps' in your gradation.
> ie 100 will be less filtration than 99 or even 98!
>
> The Aristo VC 4500 head though works differently.
> It has two bulbs, both cool on the negs (so as to not pop them)
florescents.
> But one bulb is blue for the high contrast, (same as magenta)
> And the other is Green for the low contrast, (same as yellow)
> These bulbs don't have filters passing in front of them they ARE that
color all
> the time but work on dimmers.
> So it's not a hot bulb with a dense filter in front of it.
> Its a bulb that is just not turned up so high and would have been cool
running
> even if it were turned up high.
> A much more efficient arrangement if you ask me!
> Mark Rabiner