How to determine contrast grade?

fschifano · May 9, 2006

OK, so here's my problem. I've begun to calibrate my Super Dichro II color head to my variable contrast papers. So far so good. I'm using the filter settings given by Ilford as a start and I full y expect that I'll need to tailor these settings for papers other than the Ilford offerings and my particular setup.

I have a Stouffer 31 step transmission step wedge (part number T3110) and contact print that onto a strip of paper using various filter settings. Each step on the wedge is nominally 1/3 stop more dense than the preceeding step. For each exposure I now have the first black point, the first white point, and the number of steps between those two points. It's easy enough to look up the R number range is for a given paper grade. The question is, how do I take those numbers (the number of stops between full black and full white) and turn them into an ISO R number? Is there a mathematical formula for doing this?

Stephen Benskin · May 9, 2006

ISOR is based on 0.04 above paper base and 90% of paper Dmax.

Gerald Koch · May 9, 2006

I know what I do (and I suspect many others do the same) and that is to make a print without filtration and then make an adjustment based on evaluation of the print. Unless you're into testing and calibration, and some people are inordinately fond of this, I rather be out taking pictures.

resummerfield · May 9, 2006

As Stephen said, ISOR values are the difference between IDmin (.04 above B+F) and IDMax (90% of DMax) in log Exposure. Multiply log E by 100 to get ISOR. I save much time and effort (and paper) by calibrating my system before I print.

Ian Grant · May 9, 2006

Gerald Koch said:
I know what I do (and I suspect many others do the same) and that is to make a print without filtration and then make an adjustment based on evaluation of the print. Unless you're into testing and calibration, and some people are inordinately fond of this, I rather be out taking pictures.

Yes Gerald as usual your right

Talk of Stouffer just means an obsession with the technical and an inabilty to have a quick and realistic grasph of the very basics of exposure & development in print making.

I'll qualify that by saying in the right hands any step wedge can be invaluable, but not until you are very experienced in photography & printing.

Ian

john_s · May 9, 2006

Have a look at Paul Butzi's article entitled "Variable Contrast Printing" for a calibration method including exposure adjustment as contrast is changed. If I had a colour head this is the method I'd go for.

http://www.butzi.net/articles/articles.htm

It might look very complicated at first, but it's worth contemplating.

Whether or not such testing is worthwhile depends on your workstyle and volume of work (and experience with the equipment in question, of course). I am in the process of doing a similar test using an Aristo 2-tube cold light head. I think I will find it useful since I print a wide variety of negatives in three formats, some taken by other people, and some quite old which adds to their variability. I'm looking for more volume with some of these jobs, and changing contrast without worrying about density changes would be helpful.

dancqu · May 9, 2006

fschifano said:
The question is, how do I take those numbers
(the number of stops between full black and full white)
and turn them into an ISO R number? Is there a
mathematical formula for doing this?

I think I know where you are at as I am involved in
the same sort of lab work at this time. I'm testing two VC
papers with no filtration using a condenser enlarger. I am
testing bromide's and Beer's contrast control developer
effects upon contrast. Mine is the 21 step step wedge.

In fact I've been meaning to post a question with
regard to the counting of the steps. Some are so
near black or white as to cause one to wonder
if they should be counted.

In your case multiply the number of steps by 0.1 and
check your table/tables of values. I've three. There is
some issue of the prints making as contact printing is
THE Standards method of placing grade. So if you
are after the "Official Grade' be sure to read up
on the method. Dan

theyankeesnapper · May 9, 2006

The beauty of using one format, one film, and one developer is that after a while if we conducted our film speed and film development tests with reasonable accuracy most of our negatives will start to exhibit similar densities.
I, for an example, I like to use my Jobo Vario Format Easel to pinpoint my exposure and contrast. For those unfamiliar with this easel, it accepts a piece of 8X10 paper over which it’s covered by 4 hinged light tight panels (doors). By opening each panel door separately we are exposing a 4X5 section of the 8X10. We can either expose the paper for the purpose of nailing down the correct exposure or after the exposure has been concluded, we may then expose another sheet of 8X10 paper for correct contrast with a Dichro enlarger. Normally dialing in somewhere between grades 2, 2-1/2, 3, or 3-1/2. It is also valuable in determining paper toning and dry down. Just cut the paper into 4 pieces and tone each 4X5 separately in different dilutions and times. Low tech but effective.

Regards.

Bob McCarthy

dancqu · May 10, 2006

fschifano said:
Each step on the wedge is nominally 1/3 stop
more dense than the preceeding ...

Or as a base 10 log, 10 to the 0.1 power more
dense. That is the step wedge. As for the print
it makes that is another matter. Dan

fschifano · May 10, 2006

Thanks for your replies. I've run a few different sets of test strips, and I'm not getting numbers that look even moderately reasonable. I suspect that this is a job of a reflection densitometer. Oh well, buying one is not in the budget right now so that's that. I'm not having a problem getting the sort of contrast I need. I'd just like to quantify my results so that I can have a standard upon which to build a set of parameters.

gainer · May 10, 2006

A step wedge is a negative the densities of which you know without using a densitometer. A print of a step wedge, if it has a black and a white anywhere between the ends of the wedge will tell you the density range of the paper.
If you plan to do enlargements, then use the step wedge in the enlarger as a negative. If you need a densitometer, it will be to measure unknown negatives. As has been mentioned, a very simple enlarging exposure meter that canmeasure a small part of the projected image will do to correlate the densities of a step wedge, as they printed, with densities of a negative in the enlarger.

fschifano · May 10, 2006

Patrick,

I should have mentioned earlier that I do in fact have and use the Jobo Comparator II enlarging meter. Used with the test strips, it's very easy to get a very good approximation of a given paper's unfiltered speed under my enlarger. Once you have that and the reference prints made with the step wedge using the same light source and exposure time with different filter settings or VC filters, it's easy to calculate how much exposure compensation is needed when making such changes simply by counting the stepsand multiplying your base time by the appropriate value.

Dave Miller · May 10, 2006

This all sounds very complicated. Am I wrong in producing a test strip to find the correct highlight exposure time, and then a second test strip to fix the contrast filtration that gives me satisfactory shadow tones?

fschifano · May 10, 2006

No Dave, you're not wrong. And you're right, it does all sound very complicated. In practice, It's all pretty straightforward though. It's quick and get's you in the ballpark with a minimum of wasted paper. The $100 I spent years ago for the Jobo unit has paid for itself in paper not wasted long ago.

Dave Miller · May 10, 2006

fschifano said:
No Dave, you're not wrong. And you're right, it does all sound very complicated. In practice, It's all pretty straightforward though. It's quick and get's you in the ballpark with a minimum of wasted paper. The $100 I spent years ago for the Jobo unit has paid for itself in paper not wasted long ago.

I do use a similar device; a Quadro easel that allows multiple whole image printing, 4 complete images on a 10x8. However a test strip is still best for most people, at least until they get a similar device. I was just making the observation that people seem to be making a simple task very complicated.

dancqu · May 10, 2006

[QUOTES=gainer]
"A step wedge is a negative the densities of which you
know without using a densitometer. A print of a step wedge,
if it has a black and a white anywhere between the ends of
the wedge will tell you the density range of the paper."

I detect one error. Not DR, density range, but ES,
exposure scale.

"If you plan to do enlargements, then use the step wedge in
the enlarger as a negative. If you need a densitometer, it will
be to measure unknown negatives. As has been mentioned,
a very simple enlarging exposure meter that can measure
a small part of the projected image will do to correlate
the densities of a step wedge, as they printed, with
densities of a negative in the enlarger."

I've done just that with my Ilford EM-10 Small Spot
enlarging meter. To read, all-things-being-equal, must
apply. Or as you put it "as they printed".

I do not understand the OP's step wedge count then
convert to grade problem. As for marginal steps I've
resorted to an of sorts averaging method. Dan

How to determine contrast grade?

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