To visualize and predict the look of an image before exposure it is necessary to know how your film will respond when exposed using your own personal equipment. Here is a test you can do to determine the true speed of your film with your meter, camera and lens.
First, start with an 18% gray card (the Kodak Neutral Test Card is 18% gray which corresponds to zone V of the exposure scale and print value V).
1. Load your chosen film (in 36 exposure length, or 120) into your camera.
2. Place the gray card under constant even illumination, position the camera close enough to the card to fill the frame with gray and focus the lens at infinity. (The infinity focus setting transmits light at full value and will render a featureless area of tone that will record as an even negative density) Use the second full f-stop in the range of apertures on your lens. If it stops down to f16, start with f11.
3. Meter the test surface using the film manufacturers recommended ISO setting and place that reading on step 1 of the zone exposure scale. If the meter indicates f11 at 1/15 second you would set your starting exposure at f11 at 1/250 effectively reducing the exposure by 4 stops to a zone 1 value.
4. Given the above metered value, shoot the following exposure sequence:
1st exp. 1/250 sec. @ f11
2nd exp. 1/250 sec. @ f8
3rd exp. 1/250 @ (halfway between f8 and f11)
4th exp. 1/250 @ f11 again
5th exp. 1/250 @ halfway between f11 and f16
6th exp. 1/250 @ f16
For 35mm users cover the lens and shoot off frames 7, 8 and 9 so they will be blank. Starting with frame 10 repeat the same sequence. Do this twice more to produce four groups of six test exposures with three blank frames in between each group. If you are using 120 film just do one 6 exposure sequence per roll.
In the darkroom remove the 35mm film from the cassette, bring the ends together and cut through the loop in the middle. Double each of the resulting lengths and cut them in half in a similar manner. The blank frames will give you enough leeway to create 4 strips of six exposures. Store 3 of the lengths in a light tight container and process the 4th one in your standard developer using the manufacturers time and temperature recommendations. Fix, wash and dry the film as you normally would. The 3 saved film strips can be used with other developers or if you experience processor errors.
4. Examine the processed film strip. You are looking for the test patch that has density just greater than the filmbase-fog. There are 2 ways to accurately determine this. First and best way is with a transmission densitometer. You would measure and record the film base fog density. Then find the patch that measures 0.10 density greater than film base fog.
If you don't have access to a densitometer you can make the following printing test. In the darkroom expose a piece of photo paper to white light and develop it normally to obtain a test patch of maximum black. Next place a blank (filmbase-fog) frame from the film strip into your enlarger and find the minimum exposure that produces a black that matches the previous maximum black patch. Then give that same exposure to 6 other pieces of paper through each of the 6 test strip density patches. Be sure to mark them with the appropriate frame numbers for identification. When dry compare the paper patches to the film base fog patch of maximum black. Determine which frame prints as a discernibly lighter shade of dark gray. Film base fog represents a zone 0 density so this lighter patch represents zone 1 density. If the f-stop used to create the value 1 density is the same as the normal exposure calculation (f11) then the manufacturers ISO speed recommendation is the proper one for you. If it is not the same, the degree of difference will tell you how much you must change the ISO rating of the film to arrive at the effective film speed using your equipment. So in our example, if your film was rated at ISO 400 for the test, the results would look like this...
1st exp. f11 = ISO 400
2nd exp. f8 = ISO 200
3rd exp. f8-f11 = ISO 320f8 = ISO 200
4th exp. same as first exposure
5th exp. f11-16 = ISO 650
6th exp. f16 = ISO 800
My result indicates that I need to change my ISO from 400 to 200 with this film using my equipment. Now that I know the effective film speed for my equipment and workflow, the next step is to test the entire 9 step scale. In part 3 we'll see how to determine normal, normal-minus and normal-plus development times.