in the interest of finding an Xtol alternative, I recently have taken on a short study of films in various developers. an observation is the difference in base + fog levels. 120 film, exposed, cut in half, and developed in two different developers often times yielded surprisingly different values.

after cutting the films, developing, and then splicing back together at the light table, the TMax developed films had an obvious higher gross-fog density than Rodinal or Xtol. I am curious as to why this occurs.

-- daniel taylor (lightsmythe@agalis.net), September 14, 2001

Answers

Different developing agents and formulations show a different threshold with the same film and exposure. Some developer formulae have more restrainer added to prevent fog, but this usually affects the film speed. I bet the effective speed, as measured by the exposure require to acheive 0.1D above B+F, isn't the same in all those developers, even if the gamma is made identical. It's likely that the Tmax developed film shows a higher EI than the others.
In any case, slight fog isn't a problem with B&W film. It simply lifts the overall density by a notch, and can be easily compensated in printing. It might even be beneficial in getting better tone separation in the shadows.

-- Pete Andrews (p.l.andrews@bham.ac.uk), September 17, 2001.

Pete Andrews wrote: "It might even be beneficial in getting better tone separation in the shadows".

-- Pete, how is it possible? How a _constant_ density level (ff+b) can affect the separation of the low densities in the _variable_ level (the pure image)? A sort of Art of Printing I'm not aware of? :)

-- Andrey Vorobyov (AndreyVorobyov@mail.ru), September 17, 2001.

Andrey. Film density curves start off shallow, that is, with little variation in density for a given exposure increase, and increase in slope as exposure and density increase, until they reach a more-or-less linear region where the density increase for a given exposure increase (gamma or contrast) is at a maximum.
Now, if we artificially push the shadow exposure further up the slope, then the contrast (tonal separation) will be increased, since some of the toe of the curve is now taken up by fog, and our real image detail is riding higher up the curve. Of course, it has to be a special type of fog, such as a regulated pre-exposure, or general flare in the camera.
I'm not sure if any chemically induced fogging would have the same effect, so in the context of this thread, you're right, it's not really relevant.
I think we should make a distinction between two types of fogging. Veiling fog, which is usually heavy and flattens contrast, and 'revealing fog' which actually improves the image by overcoming some of the toe inertia of the film.

-- Pete Andrews (p.l.andrews@bham.ac.uk), September 19, 2001.

I don't believe it is a traditional fogging artifact, as much as there appears to be an interaction between the acetate and developer.

-- daniel taylor (lightsmythe@agalis.net), September 19, 2001.

Pete Andrews wrote: "...if we artificially push the shadow exposure further up the slope..." -- ok, Pete, this note makes your statement clear.

There was a mess: I thought you meant increasing the fog level ONLY, all other conditions kept the same (i.e. the same exposure, therefore the same position on the char.curve, the shape, the slope and position of the very curve). -- With this assumption I did not understand how the different constant part of the density could affect the variations in variable part of the density.

Now it is all right. Thank you

-- Andrey Vorobyov (AndreyVorobyov@mail.ru), September 20, 2001.