If you haven't done it: Get a nice darkened room with one (centrally
located) window. Black out most of it, and make an array of differently
sized circular apertures - black mat-board is good enough - covered with
tape "lens-caps". Wait 'til your eyes adjust to the darkness, then open
one. The larger "pinholes" still show an image, but the "circles of
confusion" (who made that up?) are quite large and diffuse. As you go
smaller, you get a sharper (and darker) image. It's a great way to
explain all sorts of simple things to beginning students - move around a
cardboard "film plane" and show them how wide-angle and telephoto lenses
do their thing, for instance. I bet for those of you so inclined, you
could test your formulas for optimum pinhole size visually (ok, and
probably less accurately). I guess I just like to see the magic
happen...