HP5 Reciprocity failure and development contraction

I shoot with HP5 but have little experience of using it for v. long exposures. I know the reciprocity law but not sure about how much to reduce development by with v long exposures...

Any hard and fast rule about this? Can anyone help?

Cheers,

Nick.

Are you printing with multigrade paper? If so, I'd not make any changes in development unless you run out of yellow/green when printing. If you run out of yellow/green then decrease development by 20%. The graph on the HP5 datasheet ends at 30 seconds so beyond that any development compensation is pretty much trial and error depending on the duration of exposure and scene range.

If you need exposure compensation beyond the 30 sec range listed, there are some equations posted by Lee L who is one of the resident experts on reciprocity.

Thanks - good article. Found a great one by Howard Bond which Lee L recommended. Cheers