[Leica] Xtol experience

["Erwin Puts" <imxputs@knoware.nl>]

When testing the first batches of Xtol versus D76, I was puzzled by some of
the Kodak claims. To be able to compare developers, you need the same film
(obviously) and you have first to do a series of experiments to find: a) the
real speedpoint (base fog + 0.1 D) and ") the same CI value (in my case
always 0.62). I used Tri-X as the comparison emulsion and experimented as
long as needed to get identical speed points and CI values. Then I checked
first the claim that Xtol delivers finer grain and sharpness. Here I found
marginally finer grain for Xtol, but the same sharpness as with D76. The
push capabilities of Xtol were very disappointing. At EI800 I found by
densitometer that the emulsion was a full stop underdeveloped compared to
the normal speed of 400. The curves of the pushed films (800 and
specifically 1600) were steeper and gave higher density in the mid grey
areas and highlights. In the shadows remarkably less happened. In fact the
speed point moved to the right as it would when you simply underexpose.
Also I noted that the development times given were generally too short for
normal development and too regular when looking at the push steps.
After consultation with Mrs Zawadski the following conclusions could be
drawn. The differences between Xtol, D76 and even Tmax are very small and
not true for every film. Some films are better in T-max, some may benefit
when used in Xtol, some are best in D76. Room for personal experimentation!
The research goal for Xtol was primarily environmental (away from
hydrochinon) and as the BW processing is done worldwide with water of
various levels of impurity  great attention was given to buffering Xtol so
as to give consistent results in any type of water.
The second design goal was a different shape of the characteristic curve.
Normally this curve is S-shaped with a very slowly rising start, then a
fairly steep and straight part and then a rounding off at the shoulder. The
typical Xtol curve has a slightly upward curving early part (the foot of the
curve) improving shadow densities and contrast in the shadow parts and a
less steep shoulder part, making it easier to get differentiation in the
highlights. Measured curves with D76 and Xtol substantiated this claim With
Tri-X at least. Here the curves approach a straight line as Zone System
workers like it. With Delta400 the curves were straight too, but the shadow
densities did not improve (different grain chemistry here).
The push claim was the hardest part: I questioned their claim and data
sheets here. I had to present her my densitometer readings and after
discussing them , she said that indeed pushing is in fact nothing else than
controlled underdevelopment. We agreed that the times used by Kodak were
indications, based on some very thorough testing by Kodak with an accuracy
of 1/60 of a stop!,  but with some large interpolations between the test
figures. The Kodak approach here is interesting: the level of pushing
isdirectly related to the subject brightness and the desired level of
details you wish to have in the shadows. The ISO speed is not all altered by
pushing (choosing a different ISO value on your exposure meter). If your
subject has a brightness range of 3 to 4 stops around the middle grey value,
that you are interested in, then pushing may help you get a higher density
in these areas. Kodaks figures are based on this typical situation which is
indeed often encountered in available light photography.
A direct comparison with D76 and Xtol with TriX at push stage One (one stop
underexposure) gave somewhat higher density in the deep shadows for Xtol,
but NOT because of speed improvements, but because of the different shape of
the curve in the foot. Finer grain of Xtol versus D76 is not so much caused
by smaller grainclumps, but by a tighter and more homogeneous distribution
of the grain. Xtol is a definite improvement when compared to Tmax, whose
push capabilities are also a bit overhyped, but compared to D76 differences
are small, but might be significant for some workers. If you are sensitive
to environmental issues, XTol is a must.

Erwin