reciprocity info needed
i have begun to shoot alot of pinhole shots. i havea bunch of film i will use up, but i am in need of reciprocity info. i have checked the manufacturers web sites, they all say NR (not recommended) for the times i need. i no it is NR but i will need some basis to start. i would hate to spend all the time, film and money finding this out by trial and erroe. (and besides by then i would have used my stock of film)
basically info on these films would be the most helpful. porta 800, NPZ 800, porta 400UC, NPH 400, and 160NPS.
I suspect that the main reason you're seeing NR for these films is that the manufacturers assume that you want to retain accurate color balance. The different sensitized layers for RGB in these films are designed to balance each other within a specific exposure range. Once outside that range the different layers would tend to drift apart in response, failing at different rates and so causing a color crossover, or different color response/balance in the shadows and highlights, therefore not recommended for critical work.
You can determine your own rates with some relatively easy tests, a bit too long to go into here, but just a few exposures and an ND filter are required to determine a pretty useful "failure rate" for a given film. Those tests are normally done with a lens, but the reciprocity failure rate (often called the Schwarzschild exponent) should remain the same for pinhole work. Gainer's method in the thread mentioned below should also give you a practical working approach.
Search APUG for reciprocity, schwarzschild, gainer or some combination of those and you'll see some long threads on the topic, along with a couple of alternative ways of calculating reciprocity failure. For references to books with the testing methods, search APUG for Covington, Reeves, and astrophotography.
Here's one for starters: http://www.apug.org/forums/forum37/11566-reciprocity-misbehavior.html
There are also several other good ones.