It makes ISO 3200 speed in Microphen? Really? I was not aware of that.
Dear Tim,
No, it doesn't. On Ilford's own tests, it gave the absolute maximum true ISO with the original film, a fraction over 1250, but the speed difference between DD-X and Microphen is trivial.
People who know absolutely nothing about ISO tend to forget that a film might be (say) ISO 140, at which point, the manufacturer is likely to claim either 125 or 160, simply because these are commonly accepted speeds.
Most manufacturers use a middle-of-the-road dev such as, yes, ID-11 or Xtol, but (for example) I have an old Foma 200 instruction sheet which clearly shows that the film doesn't quite reach 200 even in speed-increasing developers, but it's close enough that they can get away with it.
Also, there is very slight batch-to-batch variation, typically over a range of about 1/6 stop. It used to be 1/3 stop, but all the manufacturers have got better at consistency. On top of this, as far as I recall, clamed ISO speeds are expected to be on the basis of tests with freshly-coated film and with aged film.
Ilford's stated policy at one time was to have a tolerance exclusively on the high side, in order to provide an extra buffer against over-exposure. Thus FP4 will not fall below ISO 125 in their standard test developer, but it might go to 130, 135 or even 140.
On my own tests, which are, as I said earlier, comparative and not to ISO standard, the difference between the ISO speeds of FP4 and Foma 200 is negligible. Again to repeat, I use the same camera, lens, target and lighting, and plot a D/log E curve from densitometer readings.
And despite tlidody's last observation, 'the really great thing about standards is that there are so many choose from', the simple truth is that there aren't, at least, not when it comes to film speed. As Marty says, you can use any dev you like, which gives a range of true ISO speeds, with everything else, other than the dev, firmly specified.
This is why those who know what they are talking about specify 'true ISO speed', while cheerfully conceding that quite wide variations in metering, development, etc., may mean that for a given photographer, the same ISO speed may be exposed at a wide range of EIs.
This is how FP4 can be 160+ in Microphen (close enough to be labelled 200) but well under 100 in a few fine-grain developers. And Acros 100 is under 100 in most developers. Surprise! It's also the finest-grained 'ISO 100' film. Likewise, Delta 3200 is grainier than TMZ, and also faster. You seldom get something for nothing!
Until the 1993 ISO standard revision there used to be an ISO standard developer but it was dropped when it was realized that it flattered some films and was less than fair to others. A friend (formerly with Ilford) was on the ISO standards committee at the time, and he is quite happy with the term 'true ISO'.
Cheers,
R.