Ian Grant
Subscriber
Anchell is right, Agfa and Ilford research found that once the developing agents in a diluted developer drop too low in areas where there's been a lot of development (the highlights) the developer absorbed into the emulsion gets exhausted, but they aren't in the shadows so they can keep developing. The effect is sometimes called compensating. Agfa recommended a minimum volume of Rodinal concentrate per film, regardless of the dilution to get optimal contrast and that is what Steve Anchell is alluding to.
The most comprehensive published research I've come across on exact exhaustion of a developer was done by Ilford in the 1950's, it was part of their evolution of Autophen, also known as the Axford-Kendal Fine Grain PQ developer. Essentially it's a PQ variant of D76/ID-11 and was sold to large scale photo-finishers. Unlike D76/ID-11 it didn't collapse as the Bromide built up during replenishment and one lab kept it going for years without having the restart.
That's an aside, the importance is the amount of available developing agent, so extra volume is one way of them not dropping too low. So say you use a dev 1 to 50 500ml, at 1 to 100 ideally you need 1 litre.
That exposes the reality the older days of stand development, it was done in replenished deep tanks, not highly dilute developers.
Ian
The most comprehensive published research I've come across on exact exhaustion of a developer was done by Ilford in the 1950's, it was part of their evolution of Autophen, also known as the Axford-Kendal Fine Grain PQ developer. Essentially it's a PQ variant of D76/ID-11 and was sold to large scale photo-finishers. Unlike D76/ID-11 it didn't collapse as the Bromide built up during replenishment and one lab kept it going for years without having the restart.
That's an aside, the importance is the amount of available developing agent, so extra volume is one way of them not dropping too low. So say you use a dev 1 to 50 500ml, at 1 to 100 ideally you need 1 litre.
That exposes the reality the older days of stand development, it was done in replenished deep tanks, not highly dilute developers.
Ian