Simple Step Wedge Testing
Sooner or later people in photography come to the realization that their film and paper combination may not be able to deal with a scene which is not "normal." What can be done? The simple answer is, of course, a film test. This realization is met with trepidation by some and groans by others. Get a step wedge and start testing, but how? This procedure requires a bit of time, thought and a few sheets of film, but you don't need a densitometer to do a useful test.
Basics: Film speed is a function of the amount of light hitting the unexposed film. Film speed determines how fast a film can record detail in the shadows. It is rated in asa, iso or other values. The basic idea is to have enough light strike the film to pass the threshold of exposure, below which no detail is recorded. Be careful in your tests to stay well away from reciprocity departure or your test will not be valid. Adams called this lowest value in the zone system zone I.
Basics: Highlights are the brightest part of the print. They are a function of development time. The print needs highlights which are not too dark and muddy or too light and pure paper white. The longer the development, the brighter the whites, or zone VIII and above values, according to Adams.
Basics: The step wedge is a sheet of film with known values of exposure in 21 steps. I will use a 4x5 Stouffer Step Wedge for this test. The nice thing about this tool is the arrangement of values from light to dark, each step is exactly 1/2 stop from its neighbor (0.15 units of density). This will allow us to test a sheet of film for speed, highlights and the relationship of these values against the paper of your choice in a single shot.
In general, if the film does not get enough exposure, the shadow values are too dense in the print. If development times are too short, there will not be a true white in the print. We will try to balance one against the other in a simplified test.
Testing film speed: Take a sheet of film and load a film holder. Then take the step wedge and slide it in on top of the film. An older holder which is worn is easier to use than a new tight one in this case. Find a white wall, mat board or some other well illuminated flat area. Set your camera close enough to fill the entire view, focus at infinity (no bellows extension, please). Meter the wall and you will get a zone V reading. ADD 5 stops of exposure, count it out as you increase. For example, if the wall is at E.V. 13, count out to E.V. 8 as your setting. Your first shot is a guess at film speed. A good rule of thumb is 1/2 the amount of the manufacturer's rated speed. Take the shot and develop the film at a recommended time based on your developer or your experience with the film.
Once the film is dry, look at the sheet. The step wedge is a positive, so it needs to be printed on your paper to see the relationship of black to white. For your test print, ignore the image and just do a series of bursts to get the film's edge a pure black, like the paper without any film over it. This will be your base exposure. Make the print and let it dry completely. Now look at it and find the first step showing a departure from pure black, the first step showing this change is zone I. You want the step at the bottom to be about at 19. If 18 is the first step showing a slight departure from black, you are finished. "What" you say, "How can this be?" Since each step is 1/2 stop, you are 1/2 stop too fast, cut film speed down by 1/2 stop and you are close enough to continue with highlight development.
This test result places the film's printed edge at zone 0 and the first step at 19, or zone I.
Testing highlights is done the same way. Load a sheet of film and the wedge, set the meter to give your new correct film speed and take a shot just as before. Now, remove the sheet and do three more shots the same way with the same exposure. It's time to develop the film. Pick numbers out of thin air which look to give you a zone development so that zone VIII shows up at about step 5. This is a guess, so do your first knowing the film will be underdeveloped, lets say 5 minutes. Add about 30% to this time for the next etc. (5, 6.5, 10, 13). Don't worry about zone VIII at this point, just do 4 different times which will likely be under and over. Slow film needs less time, fast film needs more time in development.
Once the films are dry, do your prints as you did for the film speed test. Examine each one and note the development time, film speed, and place a mark on the step that looks like zone VIII to you. If the step which zone VIII shows up on is step 7, you developed for about a full stop too long. If step 4 shows as zone VIII, you need about 1/2 stop more development. Don't worry about not having things exactly perfect, we're almost done now.
Get some graph paper. Make the Y axis (verticle) about an inch from the left edge of the paper. Make the X axis about an inch from the bottom. Mark these two lines in 1/2 inch increments. Along the bottom, mark starting with 3 minutes, and go out to 20 minutes on the right. Along the Y axis, start at the bottom with N+5 and continue up to N-5. This will give you plenty of leeway for a plot.
Now look at your first sheet of the four and mark the development time along the bottom axis and the guess at the correct "N" along the left edge. Lets say you did 6 minutes as your first guess, but the result was about zone VIII showing up at step 3 on the print. This is about a stop under developed. Make a mark at N-1 on the Y axis and mark the point in the field of the paper accordingly. Now continue until all four sheets are marked on the paper. Draw a line through all four points with a curve so you have a smooth line. The line won't match exactly, so try to get an average which hits the points evenly, as best you can. A French curve is what I use for connecting the dots. If you missed zone VIII development exactly, no problem, you should have a line which goes through it now, so draw a horizontal line from the "N" on the Y axis and look down at the time on the bottom. It is that simple. You should now have a good enought test to get zone VIII and other development times based on your curve.
I've left out the fact that film speed changes slightly with development time. By now you should see this on your shadow values, so make a note of your best guess at the change for this amount and write it down for each development time.
Disclaimer: I don't do this for a living. This is a very crude way of testing, when compared to a densitometer, but it will give you the information you need to get a good shot in other than "normal" lighting. There are, I'm sure, some errors in my approach, but hopefully the comments will be made to remedy these errors. tim