diluted xtol increases film speed?

From Kodak: "You can dilute XTOL Developer 1:1 with water (developer:water) for one-shot (single-use) processing. Dilution at 1:1 will provide slightly greater film speed, enhanced sharpness and shadow detail, and slightly more grain."

I understand that diluting xtol will lead to compensating effects, but I don't understand how this leads to an increase in film speed. Can someone explain a bit for me? Is this a property that all ascorbated developers share? Just how much film speed can you gain by taking dilution to the extreme?

Here is a chart of xtol dev times and speeds for tech pan (from the unofficial xtol page) that really piqued my interest regarding increasing speed by diluting xtol. Does extreme dilution have significant speed effects on other films as well? I've tried 1:3 using TMX but didn't notice anything huge for film speed. I'm wondering about 1:10, 1:30, etc,--anyone tried it?


KODAK TECHNICAL PAN FILM

Contrast Speed Developer Time
Index (EI) (70 F)
(21 C)

0.60 4 Xtol 1:2 6 min
0.60 8 Xtol 1:3 8 min
0.60 12.5 Xtol 1:4 10.5 min
0.60 25 Xtol 1:5 12.5 min

jarred_mccaffrey said:

I understand that diluting xtol will lead to compensating effects, but I don't understand how this leads to an increase in film speed.

Click to expand...

You've answered your own question here. Film speed is related to shadow detail. In Zone System terms, the film speed is related to the minimum exposure required to produce a Zone I density of 0.1. This is a somewhat arbitrary figure, but the concept is that the "speed point" is measured as a minimal significant density over film base+fog. For particular films, you might find you get results you like better with a higher Zone I density (i.e. more exposure/lower EI).

A "compensating effect" is an increase in shadow density without as much of an increase in highlight density, so compensating developers can be used to gain a real increase in film speed.

"Push processing" usually doesn't bring about much of an increase in real film speed, because when you just extend the processing time or raise the developer temperature, you increase highlight density without increasing shadow density significantly. The result is really an increase in contrast, often misunderstood as an increase in film speed.

Using dilute developer or reduced agitation or a compensating developer, though, you can really get more shadow density without increasing highlight density excessively--and that's more speed.

Thanks Dave. I get what you are saying about compensation gaining film speed via increased shadow density. Zone 1, the speed point, is an arbitrary density used to calculate film speed and if we increase shadow density we might bump Zone 1/2 up to Zone 1 and increase (double?) film speed. This can't go on forever with satisfactory results as you quickly run into the film's minimum exposure threshold ("Abandon all hope ye who enter here!"). Does/can dilute xtol in any way enhance the minimum exposure threshold?

Nothing can significantly increase the minimum exposure threshold. One stop is about the best increase in toe speed you can get with any kind of speed enhancing developer, and that's possible only because conventional developers like the reference formula D-76 aren't highly efficient at developing very lightly exposed halide.

Still, being able to take an ISO 400 film that you'd normally expose at EI 250 to get adequate shadows, and instead expose it at EI 500, still with adequate shadows and normal contrast, seems significant to me. What's a one stop increase in lens speed cost? This is just as good, and it costs about ten minutes per roll of film (and might save you a few cents per roll by using less developer)...