Understanding Film ISO Speeds

So you think you can dance?
It took me a while to comprehend ISO specifications while shooting and developing film. So, I thought I’ll give you some pointers that might be useful.

Films are rated at ISO speeds by manufacturers (there are exceptions). For example, KODAX TMAX TMX is rated at 100 ISO, KODAK TMAX TMY is rated at 400 etc. What this means is that the chemical density/composition etc allows the user to assume that eg for KODAK TMX the ISO is 100 and this can be used to calculate aperture and shutter speed settings for a given scene.

Now while developing, by changing development times you can actually change the Exposure Value on the negative (to a certain limit), as if the ISO of the film was different than what it was shot at. Of course, this is confusing.

Let’s try an example:
Using a light-meter, I metered a scene to be EV 10. I have KODAK TMX (ISO 100) and the meter calculates f-stop and shutter speed values as f/4, 1/60s (EV 10) for ISO 100. I go ahead and take the shot.

If I develop the film using 1:1 dilution of D-76 developer for 9.5 min, I’ll get the image on the negative as metered when I took the shot.
Keeping everything else same, if I develop this film for about 6 min the final image on the negative will come out about one stop underexposed … as if the ISO speed of the film was 50 (ISO 50, f/4, 1/60s = ~ EV 9).
Similarly, if I use undiluted D-76 for 11 min, the final image will look over-exposed by two stops (ISO 400, f/4, 1/60s = EV 12).

Practical applications?
Picture this (lol): It is overcast, you want to take a pic of a moving object and you have KODAK TMX (ISO 100). You meter the scene and get the EV as 10 (again??!!) which means f/4, 1/60s. You figure that 1/60s will definitely cause blur, and you need about 1/250s, plus your zoom lens won’t stop down beyond f/4 (now you know why f/2.8 zoom lenses are so costly :D)!! So, what do you do? Simple – go ahead and shoot at f/4, 1/250s (underexposing by two stops), and when you develop the film, look up the dilution/dev-time combinations for rating your film at ISO 400 and voila!!

What about the other shots on the roll that I shot at ISO 100? Go figure!

Case Study: Did you know that KODAK TMX TMZ P3200 was not rated at any specific ISO by Kodak? You can use (shoot/develop) it at ISO 400/800/1600/3200 (some have got good results with 6400 also!) … The P in TMZ P3200 stands for Push, and that means more internet research for you 🙂

Consideration: There are trade offs with graininess and contrast as you push the film during development.

4 thoughts on “Understanding Film ISO Speeds

      1. Victor Bezrukov, photographer

        Thank you for taking a look. I’m very new in analog photography so every roll for me is another lesson. I really love your deep understanding and play with time and temperature. I’m trying to stay with only 20c exactly. Will happy to read some interesting stuff from you again


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