Why is Tri-X slower as movie stock than still?

[okto simaia]

Tri-X still film is rated at 400 ISO. Why is it half that fast when packaged as movie stock?

[Joerg Polzfusz]

The „400 TX“ (aka „Tri-X 400“) is a negative filmstock, while the „Tri-X“ can also be processed as a reversal due to its nearly transparent base. In other words: These are two different films, only partially sharing the same name.

[Joerg Polzfusz]

Or to rephrase my answer:

In the beginning, Kodak introduced their first panchromatic B&W film as „X“ („panatomic  X“). Then they had a „Super-XX“/„Double-X“ (with twice the ASA of the „X“), a „Tri-X“ (with twice the ASA of the „Double-X“) and a „4-X“ ( with twice the ASA of the „Tri-X“).

But at a certain point of time (1970s?), they stopped sticking to the naming scheme.

With their remaining B&W-filmstocks for movies, the old rule is not valid anymore. (Double-X = 200-250 ASA, Tri-X = 160-200 ASA; so both films are having nearly the same sensitivity.)

And for still photography, the „Tri-X“ is now only used as an indicator to separate the „traditional“ B&W films from the newer „T-Grain“-films that are called „Tmax“.