Jeffrey is right in suspecting that there is an ideal focal length for each pinhole diameter. This chart shows the effect of pinhole diameter on resolution, both on-axis and up to 60 degrees off-axis. If on-axis resolution is most important, a user constant of maybe 1.5 in PinholeDesigner might give near optimum pinhole diameter. A slightly larger pinhole favors off-axis sharpness. Deviating by 10% from the optimum diameter causes a measureable loss in resolution. For photographers who prefer making photographs over measuring resolution, a pinhole that is larger or smaller than the calculated optimum diameter may provide the most pleasing photographs. Sharpness is academic: photography should be art.