Whoops! Sorry to muddy the waters, but ghostcount doesn't yet have a good grasp about MG filters.
The best way to learn about them is to pick a negative, one you've printed before maybe, and print at each grade. Skip the 1/2 grades, if you want. The higher grades (4, 5) will need more time than the lower ones. The you'll have a good grasp of what the different contrasts are, and how they can be used for your printing.
Split grade is much more complicated than depicted. If I can print an image at 2, why would I bother to print 10 seconds with 00, and 10 with 5. I have negs that might call for 3 seconds on 00 and 22 on 5, or I might need a lengthy low contrast exposure, and just a few seconds of high contrast. Check out Les McLean, for the low down on split grade.
I 'd suggest not going there until you are more familiar with the darkroom and straight forward MG printing.
Just in case this is where the confusion arises...
The OP appears (from his comments) to have a light source that approaches variable contrast in a relatively rare way - it emits light in varying amounts of green and blue, and adjustments are done by adding different amounts of green and blue.
Most of the information around, including information concerning the sets of filters, deals with light sources that are relatively full spectrum, and filters that subtract differing amounts of green and blue light.
It is important for the OP to think of light as a mixture of red light, green light and blue light. The red light is visible to us, but doesn't affect the paper. If the mixture of the rest of the light has more blue light than green, the print will be higher in contrast. If the mixture of the rest of the light has more green light than blue, the print will be lower in contrast. So what we need to do is adjust the contrast by adjusting the ratio of blue to green in the light.
So when he/she reads about using yellow filters, he is reading about taking light which includes lots of red, green and blue and subtracting a portion of blue (to increase the relative amount of green, and therefore reduce contrast).
And when he/she reads about using magenta filters, he is reading about taking light which includes lots of red, green and blue and subtracting a portion of green (to increase the relative amount of blue, and therefore increase contrast).
The OP's variable contrast head works more directly.
As I described on the previous page my light source works as Matt described. While there may be several different lights the one I have is from Aristo which I believe is no longer made. If he has an Aristo I would think that the controls are similar. It is quite easy and no other filters are necessary.
Jeffrey and Matt are correct this is additive mixing of blue and green light (through dichroic filters) rather than subtractive mixing. The Beseler 45 Universal VC head worked the same way.
The key for OP to get started is to know that variable contrast papers bascially have two emulsions, a low contrast layer sensitive to green light, and a high contrast layer sensitive to blue light. By varying the proportions of green and blue light reaching the paper you change the contrast. More green relative to blue = lower contrast. More blue relative to green = higher contrast.
Roughly, if you dial all the way to green you get grade 00 or 0, and all the way to blue/violet would be maximum contrast (anywhere between grade 4 and 5+). A middle setting giving approximately equal amounts of blue and green light would be around grade 2 to 2.5.
MattKing is correct. Here is a picture of my head
You can't see it in the picture, but you slide the bottom slider to the correct paper, then you dial the yellow knob left or right to select the number. The selection is a smooth transition, not a click to each varying filter. The numbers go from 0, .5, 1, 2, 3, 4, 5. In theory, I could choose something between the numbers to have a slightly different mix of blue/green than directly on 5. There is also a lever on the side to enable unfiltered white light to come through.
Thanks for all the wonderful detailed responses. I guess I was lacking a bit of understanding on exactly what the contrast numbers meant in the first place (regardless of what color the light is). I've never spent much time in the darkroom and don't even have experience with graded papers. I work in the graphic arts industry, so I have a good grasp on additive vs subtractive color theory. I suppose the Ilford manual spends more time explaining that since its much harder to wrap your head around. My head is actually much more intuitive. low number = low contrast, high numer = high contrast : )
FYI, the only reason that there is any reference to specific grade numbers is that traditionally paper had a fixed contrast, and was manufactured in several versions (Grade 1, Grade 2, Grade 3, Grade 4 etc.).
Originally Posted by edcculus
So the numbers on sets of variable contrast filters, and the conversion tables in various publications are there to assist those who learned to assess contrast while working with those fixed grade papers.
Another useful Ilford pdf is:
http://www.ilfordphoto.com/Webfiles/...0201152306.pdf, which explains how variable contrast papers, specifically Multigrade, work.
I also have a question about these filters and perhaps I didn't read things correctly. I am very new to printing and have based almost all of my knowledge on David Vestal's The Art of Black and White ENLARGING.
I don't mean to intrude on the thread at all. Also, I've read the Ilford PDF.
According to Vestal, VC paper usually=Grade 2 1/4 or 2 1/2 paper with no filter in place.
Do you folks typically find that the #2 filter prints with the same, a little higher, or a little less contrast than no filter at all, or does it always just depend on the brand of paper? I know I could just run a series of test prints but am curious to anyone's aswer. I can't seem to find an easy answer online.
Jeddy-3, the simple answer is 'who cares'. Unless you are printing a huge mural where you need all the light you can get, always use the filters. That has the advantage that you can preserve some notional mid-tone across the grades and exposures, if you use Ilford filters on Ilford paper anyway. Of course, it is unlikely that the most important part of your print will fall on that tone, but it gives you a starting point for making changes in size and contrast.
The grade numbers do not mean that you 'must' use 'this' grade for 'that' picture. They are an accident of history. Your usual negatives will usually print around the middle of the contrast range for a usual sort of image - after that, what you do is up to what you want to see. In the Enlarging sub-forum there is a sticky-thread by Bob Carnie over his printing methods (Tips from the Darkroom), which you would find interesting I'm sure.
One exercise that used to be given to most learners was to make a grid of (small, postcard sized) prints of a neg with a full range of tones that prints easily on a mid-grade. Print each grade and vary exposure by, say, 1/2 stops across a range of a couple of stops and arrange the prints on a board so that you have your 'standard' print in the middle. Going horizontally (change in exposure) or vertically (change in contrast) you see an example of the change in exposure or contrast, by whatever units you have used, and can more easily relate what you see on a future print with what you might want to change to get it where you want.
If the OP goes down the route of Ilford filters but reads extensively about the yellow being lower contrastand magenta being higher contrast such as is the case on subtractive dichroic heads using single filtration then when using Ilford filters he shouldn't worry that the range of Ilford filters do not appear to be made that way, i.e. the grade 4, 4.5 and 5 filters are not progressively darker and more magenta which intuitively you might expect them to be.
So, in short, just ignore the colours on each filter. They work exactly as they should and even if the grade 5 filter appears lighter than say a grade 3 or even 3.5 it will give a higher contrast