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“.

[Chris Letcher]

Let's not forget Tri-X 320 sheet film.

[Eric Eader]

There is the argument (since the 1960's), that TRI-X still film is actually an ASA/ISO 200 film and rating it at 400 is underexposing it.

As late as 2013 I knew a very respected Photo teacher (with extensive Military and Defense Industry Photo experience), who still argued this position.

 Virtually any B&W negative film can be processed as Reversal film, but...  that is not a rigid one method and done process.  Each film requires it's own 'tweaks' and 'massages' that vary greatly from each other. Some films don't respond well to this alteration.

 To find out why Kodak makes their recommendation, why not ask them? They are certainly in a position to know.