Response to Pinakryptol yellow in FX-2

Pinacryptol yellow is more than a simple restrainer. It's one of a group of dyes that act as desensitising agents. In other words, it reduces the film speed, and makes the film non-panchromatic and insensitive to red light.
Whether the intention was that FX-2 was to be used for development by inspection, I don't know, but that's generally the use to which PY is put.
The effect isn't immediate, but I imagine that you could examine a film developed in FX-2 by a red safelight after a few minutes immersion.
Substituting the PY with a simple development restrainer would take away this ability.

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