
On the Ilford website and various other sources, you can find a graph for determining film development time at different temperatures. This is often difficult to use since my starting time may include a fraction of a minute or the temperature of my tap water wanders off the range of the graph.
Does anyone have the formula from which the graph was derived? Thanks in advance!

Each developer I think would have a different rate of adjustment so I doubt there is a formula that exactly meets your needs. But what you might want to look at is the temps that wanders off the range. Temps effect the reducing agents at different rates so that unless you've got a specific reason for straying too far from 68deg, it's worth while to try to standardize. I don't use tap water so my bottles of distilled water normally pour out at exactly 70degs and that's my standard. I hope this helps.

The ranges presented in the graphs are the ranges where the developer will behave reasonably correctly. If your temperatures are out of hte graph range, you must change either the temperature or hte developer!

Thanks for the replies. Even within the range of graph, I find that using the graph is a bit clumsy and would prefer to use a formula if one exists.
Ole: My water temperature went out of range (26C) on a particularly hot day. I should probably try a cold water bath next time to keep it in range.
Mateo: I don't recall the Ilford site mentioning that the graph is dependent on the type of developer. I will have to check that.

I use the Zone VI compensating development timer. It has curves for both paper and film and seems to work fairly accurately in my experience.

I saw it on the NET  and remember clearly a graph was drawn form the median of Kodak and Ilford numbers...

Foundo!
Look at
http://www.covingtoninnovations.com/hc110/
It's at the bottom, near the graph.

Don: Buying a timer for this is not really an option until I setup a darkroom. It's good to know that the Zone VI timer can be used for temperature compensation though.
Jorge: Thanks for the link. I tried a few values and although some were quite accurate, others were noticeably off. I think I'll try to contact the author to find out where the formula came from.


The Ilford graphs are "one size fits all" and are approximations. (ok, I know that *everything* in photography is an approximation).
There is a simple formula that you can use if you have two time+temperature points, e.g. of the sort often published by manufacturers.
I can send you a small Excel spreadsheet if you would like it. Email me at j.sto@bigpond.com