What do the different ISO values mean on black-and-white film development charts?
Asked 1/7/2017
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On black-and-white film development charts, I see different development times listed for the same film at different ISO settings. For example, Ilford Delta 100 may show times for EI 50, 100, and 200 in the same developer.
Do these ISO values refer to pull and push processing based on how the film was exposed? In other words, if Delta 100 was exposed normally at EI 100, I would use the standard time; if it was exposed at EI 50, I would use the shorter time; and if it was exposed at EI 200, I would use a longer time?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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That's exactly what it means.
Delta 100 is an ISO 100 film, so if shot correctly and developed with Ilfosol S, the correct development time is six minutes.
If you'd overexposed the film by metering for ISO 50, you can't leave it in the developer as long, and they recommend 4.5 minutes with the same developer. This is called pull processing. (This is not something Ilford or most manufacturers recommend but is handy if you didn't set the camera properly.)
You can also push process film that was underexposed, which you'd do if you metered for ISO 200. Ilford doesn't have a recommendation for Ilfosol S, but if they did, it would be a longer time to allow more of the emulsion to react with the developer. If you look up at the Ilfotec DD-X row, you'll see that they recommend 14 minutes vs. 12 for normal development.
The blank boxes in the chart are combinations that either weren't tried or aren't recommended. (Probably the former.)
Originally by user6508. Source · Licensed CC BY-SA 4.0
user6508
9y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. Those ISO/EI values on the development chart indicate how the film was exposed, and the listed times are the recommended development times for that exposure index.
For a film like Delta 100:
- EI 100 = normal exposure and normal development
- EI 50 = the film was given one stop more exposure, so you typically use a shorter development time: pull processing
- EI 200 = the film was given one stop less exposure, so you typically use a longer development time: push processing
So the chart is not showing random alternate times for the film itself; it’s showing how to adjust development to match the way the film was rated when shot.
Development time matters because the developer needs time to penetrate the swollen gelatin emulsion and reduce the exposed silver salts to metallic silver. More or less time changes the final density and contrast.
In short: expose at the EI you choose, then use the matching chart time for that EI and developer.
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