Contrast control remains the key to making successful photographs. Contrast control begins, of course, with the scene itself, and its inherent contrast, or quality of light. The choice of film is next, with faster films being inherently less contrasty, slow film more contrasty. Then comes the film processing, in particular the development stage, which can be altered to increase or reduce contrast. Then we get into the darkroom.
Contrast means the amount of difference in tonal range in a image, from black to white. When talking about contrast in black and white photographs, terms range from flat or low key, meaning little or no contrast, to, contrasty or high key. The tones of an image are divided into darks (shadows), midtones, and highlights (lights). A flat image will have little in the way of darks and highlights, with most of the image in the midtones. A contrasty image will have most tones in the darks and highlights, with little midtones.
Contrast in printing materials is described by grade. Grades range from 00 to 5, with 00 being the least contrasty, and 5 the most. Grades are relative, with one manufacturer's materials not necessarily matching another's. But grades do give us something of a common ground, much like Zones.
Graded papers use emulsions that are formulated to produce a particular contrast grade. There are not as many left on the market as in the past, and the available grades are 2, 3, and 4 (just a quick survey of availability in the US). Grade 2 is considered normal contrast. In order to change the contrast of an image, you need to change paper grades, which means having more than a single grade available.
So in use, one would start with a grade 2 or 3, which is generally considered in the normal range of contrast. Test for exposure on the important shadows. Since the shadows are the thin parts of the negative, they will require the least exposure, and because of this determines the exposure for the image. Once the exposure is determined, one can proceed to make a full-sized test print, or alternately, make a test of the important highlights.
Making a full test print will allow you to judge the entire image. The shadows should be correct at this point, so it is the highlights we are looking at. If the highlights are too light (blown out), move to a lower grade paper. If they are too dark, move to a higher grade paper.
Simple enough. In real practice, the limited range of graded paper available (Ilford's Ilfobrom Galerie is only available in grades 2 and 3, Foma's Fomabrom is available in 2, 3 and 4), using graded papers requires some burning and dodging. This is a technique of selective exposure, using some kind of mask to block exposure on part of the image/print, while giving the unblocked part more exposure. By testing exposure for the highlights, you can determine how much more or less exposure they will require. Again, the darks should be correct already, and it will be the highlights that will need to be adjusted.
The use of graded papers has largely been supplanted by polycontrast or multigrade papers.
Multigrade (Polycontrast) Papers
Multigrade or polycontrast papers, introduced in 1940, allow you to change the contrast grade of a paper by changing the color of light exposing it. Very basically, multigrade paper has multiple layers of emulsion. These layers are all sensitive to blue light, and by adding dyes, they can be made more sensitive to green light. When the paper is exposed to blue light, all of the emulsion reacts, with a combined effect producing a high contrast image.
When the paper is exposed to green light, only the parts of the emulsion that have been sensitized to green light react. So you don not get the combined effect, and the resulting image is low contrast. (See Ilford's Contrast Control For Ilford Multigrade Variable Contrast Papers for more detailed information).
A set of multigrade filters consists of 12 colored filters, numbered from 00, which is yellow, to 5, which is magenta. Those in between are half-grades, and as the numbers increase, the magenta is added to the yellow. As the numbers increase, the transmitted light shifts from green to blue, causing the paper to gain contrast. The filters can be used in the enlarger, or under the lens. Additionally, a color enlarger head can be used, altering the color of the light by dialing in different amounts of yellow and magenta light (see the aforementioned Ilford publication).
In practice, the initial test is done with either no filter, or using a #2 or #2 1/2. No filter is approximately grade 2 1/4. This range, grade 2 to 2 1/2, is considered normal. So, test for shadows. This will give the base time for the image. Once the base exposure is determined, expose a second test sheet for that same time, with the same filtration, on a significant highlight. This should tell you if you need to lower the contrast (the highlights are blown out), or raise it (the highlights are too dark).
Retest using the new filter. Changing filters requires retesting, as the density of the filter changes. How much contrast to add or remove is something of a judgement call. That judgement takes some experience to get good at. A full grade in either direction is a good stating point. If you are adding contrast, and started with the #2, go to the #3 and test. Removing or lowering the contrast, go to the #1 and test.
Once you are satisfied, make a full test print. Some areas may need selective exposure, or burning and/or dodging. Most often it is the highlight areas that will need extra exposure, or burning. (See below).
Split Filter Printing
Split filter printing treats darks and highlights separately, taking advantage of the color sensitivity of multigrade papers. The process looks more complicated than it becomes in practice, and will get you closer to 'correct' faster than picking a filter and retesting several times.
There are, however, enough steps to the process to get it its own page.