Focal length, background control, working distance

Are you looking to get those great, out of focus, smooth backgrounds for your close ups? What lens are you using? I can tell you right now that if you're using a 50mm or 60mm macro lens that you're going to have a tough time with backgrounds, among other things.
Longer focal length lenses have a narrower angle of view. That means they also have a narrower angle of view of the background. Essentially, when using longer lenses, there's less background in your background.
Ever hear anybody say that longer lenses have less depth of field? That's not quite true. What is true is that as magnification increases, depth of field decreases. And longer lenses are capable of more magnification.

Did you know that at a given magnification and a given f-stop, all lenses will have the same depth of field? In other words, if you photograph a subject with a 24mm lens at, say f/8, then photograph that same subject, at the same size in the frame, with a 200mm lens at f/8, you'll have the exact same depth of field in both pictures. Weird, huh? It's true.
What really changes when you move from that 24mm to the 200mm is the background coverage. Well that, and working distance. But I'll get to that in just a bit. That 200mm lens has a much narrower angle of view, taking in a lot less of the background than the 24mm lens. So if you want to really control a background, use a longer lens.

In the before and after example below, the first picture ("Before") of this goofy looking little flower was taken with a 50mm lens with an extension tube to allow me to focus closer. (A macro lens has built in extension and can do the same thing). The second picture ("After") was taken at about 180mm (using my 80-200mm zoom with an extension tube) to simulate a 200mm macro.
Both images were composed so that the subject is about the same size in both, which means that the magnification is the same in both pictures. To achieve this magnification with the 50mm lens I had to be pretty close to the subject. Using my 80-200 I needed to back off significantly, bringing me back to the issue of working distance. The other big advantage of using longer macro lenses is greater working distance; you don't have to get so close to your subject. This is great when the subject is hard to get close to, is something you might disturb by being close to it (spider webs can be affected by your body heat, for instance), or is something that might run away or bite you.

The image included above was taken with my 105mm macro lens. You can see the dramatic difference between the three images. Which has the most pleasing background? Which focal length for close ups would you rather use?