True enough, for 35mm cameras. The aperture is expressed as a fraction in some notations e.g. f/8 - that's because the actual aperture is an eighth of the focal length. Small sensor cameras have shorter focal lengths to achieve the same field of view, so a compact camera will have a correspondingly smaller actual aperture, in millimeters (or whatever units you measured your focal length in), so sooner rather than later diffraction starts to affect your picture. But given the DoF that ED got, I guess he was using a camera with quite a large sensor, so not going smaller than f/8 is probably a good idea. I forget how to calculate what will be acceptably in focus at what distance and what focal length, but using an old 50mm lens in my camera bag, the scale on it suggests at f/8 and focussing at 1m, you should get a good 40cm in focus. And as ED explained, since he didn't have the light or a particularly sensitive sensor in his camera, he needed to open up the lens a bit more to let more light in, so took two shots which worked perfectly well to show me what the device looked like.