step wedge and test
I calibrate my dichroic head for new papers (actually in my case, usually very old papers) all the time - it takes one sheet of 8x10, and then my analyser gets me to a great print the first time.
I project a stouffer step wedge to about a 2.5" square size piece of paper. My Omega head goes from 170Y to 170 M in this series and for a first pass I go 30 units at a step- ie 170y, 140Y... 35Y, 0, 35M, 70M... 170M.
Label each piece with the filtration used for exposure- pencil, sharpie, etc.
Develop and process all in the same manner. Once dry, make note of the first non black step , and first non white step for each value of filtration. I use a 1/3 stop step wedge, so each step is .1 units of density.
You will find that usually the highest magenta filtration needs more exposure, to give an equivalent exposure to less contrasty filter settings. The solution is to figure in neutral density for all the settings that need it to get equal exposure. Then you can change paper grades by varying the filtration, and the highlight exposure will not change.
I look at the lowest non wihte step from the range of exposures with differing filtration, and then work out how many steps worth of ND is needed.
To do this, I measure the light value of a step that is being projected, and then measure the light output when the lens is stepped down 2 stops. Since I have a 1/3 stop step wedge, this is eqaul to 6 steps on the wedge. Yesterday, for example, I found that 12.4 seconds was the f/8 exposure.
Two stops down was 50.1 seconds. Not quite 4x, but don't worry. Now back to the open aperture, in my case f/8 Dial in C, Y and M, all to same values, until the time reading on my analyser reads 50 seconds (yes Cyan, because the analyser probe reads colour) In my case I found that it took 42 units of each to get 2 stops of light fall off. - so for me each 1/3 stop works out to 7 units. You can do this without an analyser, it just takes longer, with a series of prints needed to make the first non white when dry move 6 steps if you have a 1/3 stop wedge.
So now I know that, for instance, that first non white for me happened at 20 on the 170M, and at 14 for 0 filtration. So I need 7x6 steps of filtration to have equivalent exposure. - so my 'corrected' filtration for 0 is now 42Y 42M.
I do the same comuptations for all the other test exposures. Then I can see, for me at least, that for the paper in question, it doesn't get any softer beyond 140Y. It also tells me that without adding in extra 'offboard' filtration I will not get a harder than 4 contrtast grade. This is typical of old paper - they loose cointrast with time.
I go back to my tabulated filter data and assign which filter setting corresponds to the ISO paper range for each contrast. This data is on the sheet that comes with every type of paper Ilford ( and usually others) sells.
