What does the millimeter measurement of a macro lens mean?

Question

I understand that the millimeter measurement on a lens is, for lack of a better term, how far it can "zoom" in or out, but what about macro lenses? What does the millimeter measurement signify? How can I tell which macro lenses will be able to zoom in closer than others, since the defining factor of a macro lens is how close it can zoom in, not how far?

Answer

A macro lens creates an image on the camera sensor/film that is at least as large as the object being photographed. The ratio of the subject size on the film/sensor plane to the actual subject size is called the reproduction ratio. Good macro lenses have a reproduction ratio of 1 to 1 or higher.

For example, if you are shooting a dime, a macro lens will create an image on your film/sensor that is the same size as the dime.

The focal length of your macro lens will essentially determine how far away you can get from your subject and maintain a 1:1 reproduction ratio. Shorter focal length macro lenses require you to be closer to the subject than longer focal length lenses. Also, longer focal length macros will have a shallower depth of field than shorter focal length macros.

Check out The Digital Picture for some examples and review of macro lenses

Check more discussion of this question.