What does this switch do on my macro lens?

Question

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?

Answer

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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