Here's my method more making a Zone I (E.I. test) on one sheet of film. You'll need a darkslide marked so you can insert it in five increments. Choosing a test target can be tricky, Try to find an evenly-lit but textured surface that is not too dark. Figuring the range of aperture/shutter speed combinations you need in advance helps. You may need a neutral-density filter.
Film-speed (Zone I) Test
First, determine your starting exposure, (usually the rated film speed, or the highest probable E.I. you plan to use). Meter the test target and set shutter and aperture combination. Use a rather slow shutter speed, they are more accurate.
Then, pull the dark slide completely and make the initial exposure. You are exposing the entire sheet to Zone I at your highest-likely E.I.
Now, for +1/3-stop increments in descending film speed, do the following:
1. Close down 2 stops, insert the dark slide part way (1/5), and make the second exposure. You've now got one stripe at your highest E.I. and the rest of the film at an E.I. 1/3-stop faster. Continue exposing now in 1/3 stop intervals...
2. Open 1/3-stop (use aperture here; a lens with 1/3-stop intervals marked helps, otherwise, estimate), insert the dark slide another 1/5th and make the third exposure.
3. Repeat this procedure for additional exposures on the same sheet of film, i.e., insert the darkslide another 1/5th, open 1/3-stop and make an exposure. Do this till you've used up the entire sheet.
You will now have five "stripes," each exposed 1/3 stop more than the previous one. So, if you started with, say, E.I. 400, you'd have the following sequence:
400 - 320 - 250 - 200 - 160. That's a one-and-one-third-stop spread and should give you enough to determine a working E.I. for a particular film and developer.
You can then read the negative with a densitometer to find your target Zone I density or use the following visual method to find your personal E.I.
The visual method is as follows:
Print your test negative however you're going to print, i.e., contact print it if you're going to contact print, or use an enlarger if you're going to enlarge. Make a test strip of the negative along with a stripe of paper that is exposed to light not coming through the negative (if you're enlarging, you'll maybe have to trim the negative so you get a stripe of white light along one edge of the negative, or use a larger negative carrier, glass carrier or whatever.)
What you are looking for is the minimum exposure that renders the clear rebate of the film almost exactly (but not quite) as black as the black of the paper exposed to light not passing through the negative. Some say the blacks should match, but I think that gives a too-slow E.I. Anyway, evaluate the dried test strip under average (not too bright or dark) lighting and find that print exposure. Then make a print of the test negative at that exposure time. Dry the print and examine it to find the first stripe that is noticeably lighter than the black of the film rebate. This is your proper Zone I and the corresponding exposure index is your effective film speed. When in doubt, use the next slower E.I.
Keep good field notes and adjust if needed.
Or, you can save yourself the test and try this: Rate your film 2/3-stop slower than box speed and go shoot. Keep notes and evaluate your shadow detail from the negatives you get carefully. If shadows seem more fully-exposed than needed, adjust your E.I. 1/3-stop faster and keep shooting. Do that till you get shadows without as much detail as you like and then go back 1/3 or 2/3 stop slower for your personal E.I. This is what I do anymore...
Have fun,
Doremus
