In what I consider the "bible" of pinhole Photography (PINHOLE PHOTOGRAPHY Rediscovering a Historic Technique by Eric Renner) is a list of the optimal diameters for pinholes for focal lengths ranging from 10mm to 1,000mm. I've noticed that in the current line of the various pinhole cameras offered on the EBay, the pinholes are usually a lot smaller than the recommended optimum sizes. Purchased an 8x10 pinhole camera in which the pinhole was a whole lot smaller that the recommended size. I enlarged it to the recommended size and my negatives appeared "sharper". I'm trying to get to the point of where my images look like "sharp" pinhole images... but definitely not images taken with glass optics.

My pinholes are made by drilling pinholes in .002" Brass Shim Stock. I also darken the sides of the pinhole aperture with a black Sharpie. Definitely cuts down on reflections from the raw cut/drilled brass stock on the outsides of the drilled hole. Lately have tried drilling pinholes in the aluminum soda cans are made of. It seems to be thinner than .002" but don't have an accurate measuring device to confirm that.

Would love to know other's experiences in achieving optimal pinhole sizes.