Quote Originally Posted by nhemann View Post
scan it at the highest possible resolution you can (mine is like 2000 dpi) enlarge that image in a graphics program - Gimp is free - and then select it with a circle tool to tell you how many pixels wide it is. Multiply: number of pixels * inches-per-pixel and voila - pinhole diameter!

I remeasured mine with a microscope once and the method was pretty damned accurate.
What he said!