I see no weakness in your purchase. Have fun with it. There is always a little reciprocity failure with all film at such long exposures, but don't worry, a little overexposure of Type 55 never hurts. I don't use any formulas - just trial and error. At most I've ever had to extend an exposure is about a minute for a 5 minute exposure. In the 10 secs to a minute exposure range, the RF is in the seconds in my experience.

I use a "Black Cat" exposure guide. Most meters don't have an f/stop range of a pinhole camera. I take an exposure reading, say 1 sec at f/64. I set the guide at that reading. Then I go to the aperture of the pinhole, say f/256, and look up the new exposure time - 30 sec. Easy as that.

All you need to do it look up the f/stop number of your pinhole body cap. My Nikon pinhole body cap is f/185.

Hope this is helpful. My web site shows a lot of pinhole photography (www.walterpcalahan.com) if you go to the experimental button on click on PinholeDC.

Quote Originally Posted by Magpie

In a weak moment I purchased a Mamiya Body Cap Pinhole and a 'Pinhole Factory Exposure Calculator' on ebay.

After reading the various threads here I have rough idea how to take some pinhole photos with my Mamiya, and given that I have a few packs of soon to be expired Polaroid colour & B&W film I was considering experimenting.

With the colour polaroid is there a reciprosity factor I need to add to my exposures?

Also can anyone explain how the #$^&*( the Exposure calculator works? It has time on the outer ring and f numbers on the inner ring. Is there something missing - the pinhole size? or am I missing the point entirely?

I will be getting a book from the Pinhole Resouce centre but wanted to try some first experimants this coming holiday weekend - 4 days off work!