Life-Sized Images

• The purpose of a macro lens is to create images that are at least as large in the image as they are in real life. The size of the subject in the picture compared to the actual size is the magnification ratio. A macro lens should have at least a 1:1 ratio, but nicer lenses will have up to a 5:1 magnification ratio.
  
  

Aperture

• The aperture of an SLR macro lens is crucial, depending on the types of images you will be shooting. If you are taking pictures in low light, or shooting fast-moving objects like small insects, it is critical to have a wide aperture--2.8, or less--in order to use a shutter speed that's fast enough to stop the action.
  
  

Focal Length

• Single-lens reflex macro lenses range from 50 to 200 mm in focal length, which affects the lens' field-of-view. If you are shooting very close to your subjects, 50 mm is sufficient. However, if you can't get close, perhaps because the insects will fly away, you will need a longer focal length.
  
  

Minimum-Focusing Distance

• The minimum-focusing distance is how close the lens can get to the subject and still focus on it. For example, if the minimum-focusing distance is 6 inches, the lens would need to be at least 6 inches away from the camera in order to focus on it. The shorter the minimum-focusing distance, the better.
  
  

Extension Tubes

• Macro lenses can be expensive, sometimes prohibitively so for people who just want to test out macro photography. Extension tubes are much cheaper, but sacrifice some image quality. The hollow tubes extend the distance between the image sensor or film and the lens itself to reduce the minimum-focusing distance.