Excerpt from Scott Kelby's Digital Photography book, Chapter 3—When Should You Use a Macro Lens?
This is the lens you reach for when you want to photograph something really, really close up. You have probably seen a close-up of a bee, a ladybug, or a flower, right? That is macro photography. Macro lenses do only one thing, but they do it extremely well. There are a few things you need to know about macro lenses.
(1) They have an amazingly shallow depth of field—so shallow that when you photograph a flower, the front petal may be sharp while a petal on the other side of the same flower is so blurred that it is barely recognizable. That shallow depth of field is one of the things I love about macro lenses. At the same time, it also makes it difficult to get several things sharp in the frame. (Try shooting at f/22 to get as much as possible in focus. For a little more depth, also keep your lens level and avoid shooting upward or downward at an angle.)
(2) Any movement or vibration produces a blurry photograph. Use a tripod whenever possible. Use a cable release so you do not have to touch the camera and introduce additional vibration. (You can read more about cable releases in Chapter 1, 'Tips for Taking Sharp Photographs.')
