There is a switch on my Canon 100mm USM Autofocus Macro Lens that switches between 0.32 mm and 0.48 mm (I think those are the numbers I dont have my camera near me). What do these numbers mean and in what situation would I use either of the two options?
Those numbers are in the wrong unit if you are taking about this lens. It should be in meters.
That is the minimum focus distance switch. It prevents the lens from focusing any closer which speeds up autofocus. Some telephoto lenses have it too.
You should set it to the highest number unless you need to focus closer.
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Autofocus on a macro lens is very sensitive, so it's a cue to the lens in what range you expect to find your subject. It's a focus limiter. When set to close range, it limits autofocus's search to edges (that's what autofocus tries to find) in that short range, reducing the chances of really random focus search. If, however, you are doing a portrait, set it to the full focus range so it can "look" further. It won't autofocus as well as a macro, but it will find edges at a normal distance.
I really recommend manual focus when doing macro work, as you know your depth of field and what is important in your subject, whereas your autofocus doesn't.