Ok, here's the camera and first photo
Here's the camera I made using the Tektronix C-50 series Polaroid pack film back:
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I took this brass light stand adapter stud thing, drilled some holes in the film holder, then bolted it onto the holder using a 5/8" bolt and some washers to distribute the force. The other side on the adapter is a standard 1/4-20 tripod hole so it bolts on nicely to the arca-swiss compatible plate that came with the ball head for my Gorillapod. I glued that wide fender washer to the tripod side of the stud with some silicone glue. It should help distribute the force on tripod plates, stabilizing the camera more and avoiding damage to tripod plates that don't deal well with the small base of the stud.
The Polaroid back also has a slot for a darkslide, but the actual darkslide was missing. So I made a new one using, guess what? The metal front plate from an old metal Polaroid cartridge! Though the darkslide was no doubt originally intended to allow you to switch backs on your osciliscope camera without exposing the film, I decided that I'd just use it as a shutter for the pinhole for now.
I used the Gorillapod because my real tripod is sort of occupied at the moment. Then I just created a really simplistic test photo setup like this and tried to align the camera best I could:
The pinhole is supposed to be f/276 and since my light meter only lets you set it no smaller than f/90, it seems that I essentially have to divide the film's ISO by 10 (how convenient) in this case and use that as the ISO in the light meter. Since I don't want to wait forever for this low light photo I put some Fuji FP-3000B in the thing.
The light meter said something like 2.6m and I had to guess at the reciprocity failure for such a long exposure. I guessed +3 stops, so I estimated the exposure at about 20 minutes. Looks like I guessed right on the first try. Here's the whole print from the shot, "scanned" using a "scanner" iPhone application (one of those ones that corrects for perspective distortion etc):
That's with no adjustments other than the scanner app's guess at the correct contrast.
I'm pretty happy with that, expect that I'm wondering where the heck that black line came from. It seems like something was covering the negative the whole time, somehow. I'm wondering if it just got misaligned in the cartridge or something when I pulled the masking paper out of the new film pack. I can't really think of anything else that might cause it except maybe I didn't fully push the film pack in or something, or it's a random film alignment problem. I guess I'll just see if this keeps happening.
Now I just need the sun to come up so I can take some real photos!