How to find the true film speed

Resident film expert Martin Zimelka discusses finding your film’s true speed:

This is a short how-to guide for the modern B&W film shooter who scans his negatives. There is nothing here that hasn’t been repeated before, as it’s derived from techniques and methods far older than I am. It’s definitely a technique worth repeating, because I believe it to be invaluable with regards to understanding different film, and getting the best out of it.

The first thing you’ll have to do is find a neutral, mid-toned, and detailed subject in even and unchanging light. For example, a concrete wall, or as a friend suggested, a towel will do fine as well.

Remember to choose a subject that is adequately lit, one that will offer sufficient light for you to stop down 4 stops without running into the limits of your camera/lens settings, or into film reciprocity failure. Avoid shooting the lens wide open, especially with fast lenses for the 35mm format. It’s preferable not to change the shutter speed if possible, unless one is using a modern SLR like a Nikon F5, which has accurate, self-diagnosing shutter speed times throughout it’s range. Use the camera in manual mode, and make careful notes of everything you do!

  • First take a meter reading at the film’s box speed, and note it down. This exposure would render your “concrete wall” a neutral grey, and a theoretical Zone V
. Take a picture if you like, but it’s not critical.
  • Now, stop your aperture down 4 stops, which should theoretically give you Zone I. It’s very likely that this won’t be the true Zone I for the film, since manufacturers often claim box speeds higher than the film’s true speeds. Take the exposure for this theoretical Zone I, but also take a series of additional images, each with the exposure increased by 1/3-stop increments. An additional six (6) frames in 1/3-stop will eventually allow for a 2-stop margin for film speed error, and if you are shooting 35mm, I suggest doing it this way.  So now, somewhere in between those 7 frames, you’ll have found the true Zone I. It may be that your lens or camera doesn’t support 1/3 stop increments. If so, use half stop increments.
  • Take a few exposures with the lens cap on, which should give you a theoretical Zone 0, which you will use as reference later on.
  • Finish your roll on whatever you may desire. It is suggest one finishes the roll, since some photographers claim that an unfinished roll can effect developer activity due to the lower than average levels of exposed silver on the film.
  • Develop your film as per standard recommendations, or the way you usually do.
 The important thing is to maintain consistency.
  • After your negs are dry, scan them, and compare the Zone I images with one of the blank Zone 0 frames. The proper Zone I should be the next step up from Zone 0, where the first sign of detail should be discernible.
    Once you have found the frame with the true Zone I, take a look at your notes, and calculate how many stops this frame was overexposed from your theoretical Zone I exposure.
    The difference indicates how much less your true film speed is to the advertised box speed. 

Example: Lets assume your film has a box speed of ASA 400, and your Zone V exposure was 1/125 sec f/5.6. Your theoretical Zone I exposure would have been 1/125 sec f/22. However, your scans show true Zone I to be one of the bracketed frames you exposed for 1/100 sec f/16. The difference between the theoretical and practical Zone I exposure is 1 and 1/3 stop. Subtracting that from your the film’s box speed, will give you a true film speed of ASA 160!
  • Now that you’ve determined your true film speed, you’ll have to reduce your developing time, otherwise exposing for ASA 160 will cause your mid tones to be too bright, and your general contrast to be too high.
    As a starting point, reduce the developing time by 33% for every stop your true film speed is slower than the box speed rating. Then, with your next roll of film, determine if the mid-tones have a density to your liking. If not, adjust developing times.
    Controlling the agitation intervals will give you more control over your highlights. Less agitation will reduce highlight density.

To end this off, I would like to point out a few key variables.

No light-meter is the same, and no lens is identical in light transmission. It’s important to understand this, and therefore it’s important to do this test with the equipment you use the most. I use my handheld meter the most, so I would conduct this test using my hand held light-meter.

Also, each film is different. Some films give true film speeds, some don’t. As an example, Ilford Delta 3200 is nowhere close to being an ASA 3200 film, where as Efke 50 is, or better said was, a true ASA 50 film (developer dependent). You’ll have to conduct this test on each film type you use.

Finally, some developers reduce film speed, others don’t. Some push developers are not really true push developers, since their apparent increase of film speed comes at the cost of killing off the lower zones. It’s important to test your film’s speed in each developer you use.