The aperture (diameter) of a telescope determines how much light it picks up. A 10-cm (4-inch) telescope picks up only a quarter as much light from the same celestial object as a 20-cm (8-inch), because a 10-cm circle has only a quarter as much surface area as a 20-cm circle.

Telescopes are rated for their aperture; camera lenses, for their focal length. Thus a 200-mm camera lens is much smaller than a 20-cm (200-mm) telescope.

In this book, apertures are always given in centimeters or inches, and focal lengths are always given in millimeters, partly because this is traditional, and partly because it helps keep one from being mistaken for the other.

In the context of camera lenses, "aperture" usually means f-ratio rather than diameter. That's why I use the awkward term "aperture diameter" in places where confusion must be avoided.

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