"Now this was really interesting to me, because I had always thought that a pinhole was just that, that you could only get an image with a little tiny hole. It turns out that you can have any size aperture, and any shape, and as long as you get back far enough with the screen, it will resolve itself into just about as good an image of the sun as you get with a little pinhole close up.
"One way to see this is to use just the square hole. You can take just this little tiny comer of the square hole and make a little tiny image with the light coming through that corner. Now about a quarter of an inch away, there's another tiny image coming through the other corner. So now there's an image there, and an image there, about a quarter of an inch apart. But now, as I pull back, both images get bigger and bigger.
"But their centers are no farther apart than that - about a quarter of an inch, so by the time the images have gotten real big, the centers are effectively in the same place. So you have a single large image of the sun, with a quarter-inch fuzzy edge. The limit of the resolution is the size of the aperture.