Thanks for all the good advice here.
So given that I'm using 4.25x3.25" film, does the mrpinhole calculation of 136mm and 0.49mm seem optimal? From reading something elsewhere, I seem to remember that his calculator's assumption of ideal is "maximum sharpness" vs. "maximum resolution". This actually has me wondering if it would be better in both softness and sharpness terms to optimize for resolution and then use sharpening algorithms after scanning if sharpness is that important. (In other words, will "better resolution" images sharpen better digitally while being higher resolution than "better sharpness" images would be in terms of both?)Pinhole size and quality is only one factor affecting overall resolution, another major factor being format size. For equal angles of view, a larger film format will require a longer focal length and thus (by Rayleigh's formula) a commensurately larger focal ratio, which will produce images less affected by diffraction. There is also the advantage afforded by larger film formats of needing less enlargement, further reducing image degradation.
I've thought about just buying electron microscope apertures, but I figured that I'd try doing this myself first. I'll see if I can find some suitable brass. (Wonder if the hardware store has brass stock that thin?)But if you're going to find the optimal pinhole, and don't want to purchase commercial electron microscope apertures (which are the best one can find) then do yourself a favor and choose a material a bit better suited than soda can metal. You want to make your pinhole as close as possible to a two-dimensional hole, not a three-dimensional tunnel. The more tunnel-like your pinhole, the more off-axis vignetting and loss of resolution, especially if you are after wider angles of view.
Speaking of film and long exposures, it kind of seems like this Fuji FP-3000B stuff has a pretty bad tone curve for long exposures. The chart in the following photos, when photographed using a Mamiya 645 using the same FP-3000B film with a polaroid back and relatively "normal" exposure times, will show a clear difference between all of the patches when the middle gray one (big patch in the middle) is exposed properly. Each of these patches is +/- 1/6th EV from the ones next to it.
However the same chart photographed with my pinhole using the same FP-3000B with a 30 second exposure (the light meter said 15s and I added a stop for reciprocity failure) looks like this:
(I had the camera on a little tripod on the carpet and was sitting and moving around a foot or two away, so I wasn't exactly expecting sharpness here, just an exposure test.)
Then again maybe this particular film is just more sensitive to development time than the FP-100C. (It's actually supposed to be 15s for 80F rather than 30s.) Fuji's newer films are supposed to have "self terminating" chemistry or something, but I haven't really looked at it carefully with the FP-3000B. Anyway, to me a lot of these patches don't look very distinct. (They seem to look more distinct in this cropped but otherwise unmodified photo of the print, but something in the iPhone OS might be applying some sort of adaptive tone curve when processing the sensor raw or something.)