Film is not really adjustable in speed. When the
developing time is increased or decreased there is a change
in the _effective_ speed, which Kodak calls the Exposure
Index, its as good a term as any. Now, changing the amount
of development mostly affects the contrast. As time is
increased the high exposure areas grow in density faster
than the low exposure areas, however, all increase to some
degree.
  The ISO speed is measured at a fixed contrast. Actually,
the standard specifies a range of exposure stated as a log
ratio to obtain a log ratio of density with the lowest
density specified as being logE 0.1 above gross fog and base
density.
  If the film is developed to a lower contrast than the one
specified in the standard it requires more exposure to
obtain a minimum usable density in the shadow areas.
Conversly, if the film receives more development some
reduction of exposure is possible because the shadow areas
will be somewhat denser than they would be for normal
development. Reducing exposure is not absolutely necessary
but helps to avoid overly dense highlight densities.
  The amount of change in development time to effect a
given amount of contrast change depends on the film and
developer. Conventional films, like Tri-X, will change about
one paper grade when development time is increased or
decreased about 33% from the "normal" time. The variation
depends somewhat on the developer, but is mostly a
characteristic of the film. Tabular grain films like T-Max
will give the same change in contrast with only about a 25%
change in time.
  The speed difference will be about +/- 3/4 of a stop for
this much contrast variation regardless of the film.
  Now, not all developers deliver the same speed at normal
contrast. At one time the ISO standard specified a developer
but it was complained that this developer bore little
relationship to those used in practice, so the standard was
changed to allow the use of any developer, provided it was
specified along the the resulting speeds.
 The variation in speed with developer types is also around
+/- 3/4 stop. Some Phenidone developers yield about 3/4 stop
greater speed and some developers, notably Rodinal or
Microdol-X full strength, loose about 3/4 stop. Both of
these vary with dilution. D-76 has become the defacto
standard developer for determining speed and the
comparisions above are in relation to it.
 If the Tri-X you mean is the ISO-400 variety (roll and
35mm film) EI-280 is about 70% of the rated speed so one of
the speed loosing developers will give you this speed with
normal contrast.
 Now something about exposure. The ISO standard yields
about the very minimum speed that will give good shadow
detail. Actually, many photographers find that increasing
the exposure, which is the same as shooting the film at a
slower speed, gives better shadow detail. Because the film
is tested for the minimum exposure for good tonal rendition
there is lots of latitude on the increased exposure side and
almost none on the reduced exposure side.
 I've gone into this detail because I think its not very
well understood. In fact, if you want to use Tri-X at EI-280
you can develop normally in any developer but the negatives
will be a bit dense if you use a speed increasing developer
like Microphen or Xtol, and will be pretty normal if you use
D-76, HC-110 or Rodinal.
 A clue to the amount of latitude is that the pre 1960 ASA
standard had a 2.5 X safety factor built into it. The
current ISO standard has a 1.25X factor, so the old speeds
were just half of the same film now. At the time the ASA
speeds were generally considered to produce excessively
dense negatives and it was common practice to use less
exposure. Now we have a standard with minimum exposure and a
lot of people increase it.
 To make the point, shoot some tests at bracketed exposures
about 1/2 stop over a range of about 2 stops more and less
than the film rating. Develop normally and see what you get.
That will give you a very good idea of what sort of tonal
rendition you will get for a given exposure index.

In general and within a limited range increasing exposure affects mostly
shadow detail, more exposure with Tri-X yields better shadow detail.
Processing times should not be altered from your normal time unless you also
need to adjust overall scene contrast.  In my experience Tri-X is very
forgiving or overexposure errors and I usually expose it at EI200 and may
expose at EI100 for scenes with short contrast ranges.  If you have a long
exposure range you can push your highlights up onto the shoulder area of
Tri-X's exposure curve and reduce highlight contrast.  Useful for those
black and white shots of black tuxedos and white wedding gowns.

As an experiment I'd suggest shooting frames at EI 800, 400, 200, 100 and 50
on both a roll of Tri-X and a roll of TMax 400 and process both rolls in the
same small tank at you standard time for either film (should be fairly close
developing times anyway) and then make the best print you can from each of
10 negatives.  Then subject the prints to a blind comparison from ten
knowledgeable photo buddies and ten tyros.  Have each of them rank the 10
from best to worst.

--
darkroommike

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