Response to More on film speeds

Thanks everyone.
I found the answer in a textbook that I hadn't looked at for years.
For future reference, the official ASA rating was derived by using K = 1.25, or 0.8/(lux-seconds). Bill C was bang on.
That's the 1962 ASA standard, which I think still holds as the current ISO standard.

It does seem that the recommended ISO development gives a curve that most people would find too steep for their liking. In effect, it appears that the ISO speed is derived from a slightly pushed development.
It also means that colour negative film can't be given a proper ISO rating, since its development is standardised away from the ISO recommendation.

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