Hey -- all pinhole is good! (Especially if you get recognizable images!) In my opinion, the f-number is an indicator, but not necessarily the design target when talking sharpness. The optimum pinhole is affected by the pinhole to film plane distance, and also the size of the film format. The effective pinhole size varies a bit off the central axis which contributes to exposure fall-off toward the sides in a wide angle camera -- the effect can lead to flat-out vignetting if the pinhole plate material is relatively thick. I note some background details on the shot above, those along the left and right edges, appear a lot sharper than the flowers in the central area -- so perhaps that's saying try a smaller pinhole (or maybe a breeze was wiggling the flowers?!)
You haven't said (unless it's in another thread) what film format and dimensions you're using here. I've typically wound up with f-numbers around 210 to 275 or so on the cameras I've built, based on using the Pinhole Designer utility to do the calculations. There is a magic constant used in those calcs that defaults to 1.9, attributed to Rayleigh, but I know Jim Jones, who's an active pinholer here leans to a smaller constant, like 1.4-something. I used 1.65 in my last effort. So far I make the observation, the larger the film format, the sharper the result appears. I've had 8x10 contact prints from pinhole shots on x-ray film wind up in an exhibition or two. They don't have the "snap" of something from a Tessar lens, but they can be pretty amazing when considering there's no lens involved at all!
You can check out some of my good, bad, and ugly through
these galleries. I mostly do a big (or not) once a year extravaganza on the last Sunday in April, Worldwide Pinhole Photography Day.
