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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?

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The instruction manuals for Canon lenses are available online. Yours is here. See the left side of p. 5. – whuber Jul 20 '11 at 20:08
possible duplicate of What is a focus limiter? – Matt Grum Jul 21 '11 at 16:51

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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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Yes this lens but mine is not IS (not sure what that means) – maq Jul 20 '11 at 17:07
And so if I'm really close to a subject and I want to focus on it, I should set it to the lower number? – maq Jul 20 '11 at 17:07
IS = Image Stabilization. It compensates for tiny involuntary movements of the photographer. – Itai Jul 20 '11 at 17:08
Yes, obviously. It refers to the distance from the sensor to the subject, which is almost at the back of the camera. – Itai Jul 20 '11 at 17:10
On Nikon SLR bodies, the position of the sensor is indicated by some subtle marks on the body, and the center of the tripod mount point is below the intersection of the optical axis and the sensor image plane. I assume there are similar properties of Canon bodies that are either documented or can be discovered. – RBerteig Jul 20 '11 at 21:06
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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.

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Might be worth pointing out in-case its not obvious, that the further the camera has to look/hunt to get in focus the longer it takes to focus correctly. – ChrisFletcher Jul 20 '11 at 17:41
Yes @chrisfletcher, that's true, and with this particular lens, unless limited it may not find focus. The barrel moves when focusing and it extends a long way, which is time consuming and you can find yourself cussing the thing out when in fact using the limiter or manually focusing may be more effective. – Steve Ross Jul 20 '11 at 18:07
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The focus limiter actually works in the opposite sense suggested here: the 0.48m setting limits the range to 0.48m and further (to infinity); the 0.31m setting gives the full range from 0.31m to infinity. One would have to use the latter (which is the "full focus range") only for autofocusing close-ups. – whuber Jul 20 '11 at 20:11

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