Is there some quick rule of thumb for Reciprocity Failure for B&W film? I went out with my 4x5 at early evening and made some very thin negatives. (Photo is one of the less thin negatives.)
It depends on the film. Some films aren't so bad but some like the cheaper Fomapan 4x5 films are a pain for long exposures. If you have an iPhone there's an app called reciprocity timer that you can put your exposure value into and dial in adjustments for things like reciprocity, filters, bellows extension, etc. Very handy. I'm trying to find a similar android app but can't find one.
Most films average out to adding 1 full second to every 15 timed seconds. Mathematics and arithmetics and wizardry can help you figure out what times change to what, and when.
Or you could look for official documentation on the failure rates from the film manufacturer.
Whichever is easier.
That is completely incorrect. You're implying it's a linear relationship and that's entirely false.
You don't just add 1sec every 15sec. You need exponentially more exposure as the metered exposure increases.
See this example of HP5:
metered (s) = corrected (s)
5 = 12
10 = 31
15 = 56
20 = 85
Forgot to add...
OP, as I explained in >>2764624 this relationship changes depending on the film being used. Some films handle reciprocity really well and you can easily find charts for them. It's easiest to stick to these films for really long exposures as it's easier to get the exposure right and know how the film will react (I do this for LF as it's way to expensive to shoot lots of bracketed 8x10 shots). A lot of the cheaper films don't handle long exposure well at all. For a lot of them the manufactures wont provide the data and because most people don't use them for long exposures you wont find much 3rd party info for them.
Hope that helps.
Wait now I kind of remember what it was; something about adding seconds to bulb exposures, which digital cameras are (or used to be) programmed to do, for whatever reason.
Fuck it. I still don't remember very well. Disregard everything I've said.