The EI Method

Everyone has had the experience of setting the wrong film speed on the camera or meter and subsequently over-exposing or under-exposing the film. For instance, if you are using a film with an exposure index of 100, but set your film speed to 200, you will be underexposing the film by one stop. If that were the case and you followed the meter recommendation, thinking that you were exposing a subject on Zone 5, you would really be underexposing by one stop or exposing the subject for Zone 4.
For the same film, if you set the meter for 400 and exposed according to the recommended meter reading, you would actually be underexposing by two stops or placing the subject on Zone 3. If you set the film speed for 800 you would be underexposing by 3 stops or placing the subject on Zone 2. If you set the film speed for 1600, you would be underexposing by 4 stops or placing the subject on Zone 1.

On the other hand, if you set the film speed for 50, you would be overexposing by 1 stop or placing the subject on Zone 6. If you set the film speed for 25, you would be overexposing by 2 stops or placing the subject on Zone 7. And if you set the film speed for 12, you would be over exposing by 3 stops or placing the subject on Zone 8.

Neat. You can simplify some of the manual counting of stops, by selecting the film speeds which appropriately place the metered subject on the desired Zone when the meter is followed exactly. This is not a practice recommended for use making photographs, but it is a handy timesaver when doing film tests. It also allows some automatic cameras to fully participate in film tests. Here is an example:

Compare the sequences above to "example 2" from a couple of pages back:

To conduct a film test using the chart above, you would first make the two preliminary exposures, one with an opaque lens cap on to establish film base plus fog (fb+f) and another of a sheet of paper with a written description of the current film test. Next you would make the five Zone 1 exposures. Assuming that you are using a diffuser, instead of a neutrally colored evenly illuminated reflective subject, set your first filmspeed for the 2/3 stop more light EI for Zone 1 -- EI 1000 in the chart above. If possible, considering the amount of light available, set your shutter speed on 1/60. Meter through the diffuser and set the f-stop to the setting that the meter recommends. Expose. Now set your film speed to the 1/3 less light EI (EI 2000 in the chart) keep the shutter set at 1/60 if possible and expose according to the meter reading. Next repeat with the 1/3 more light EI for Zone 1. Then repeat with the 1/3 less light EI for Zone 1. Then repeat with the target EI for Zone 1-- EI 1600 in the chart. Whenever there is a way to do it, keep your shutter speed on 1/60. If you are using a diffuser, you can sometimes keep the shutter speed constant by inserting neutral density filters into the diffuser when you have run out of f-stops by need to reduce the amount of light reaching the film. Just be sure to meter with the diffusers in place.

After finishing the (5) Zone 1 exposures, proceed to the highlight exposures. Do the same sequence as before, start with 2/3 over exposure (EI 8 in the chart above), then 1/3 under (EI 16), then 1/3 over (EI 10), then 2/3 under (EI 20) then the target exposure (EI 12).

That's it. You're done with this instance of the test. Develop the film, read the densities, interpret the results and do another test. The next page provides a mechanism for calculating the EI table above for many film speeds and for N+2, N+1, Normal, N-1 and N-2 developments.