You should understand that the focal length of a macro lens determines how close you have to be to an object to render it in your frame. For example, you might have to work 50mm from an insect with a 50mm macro, vs. shooting from 100mm from the inselt with a 100mm macro lens. However, shooting distance is not the only determinant in choosing macro lenses, because how close you can focus will be dependent upon the lens design, and you might be able to fill the frame with a larger image of the insect with one lens but not the other (unless you resort to using extension tubes to permit you to focus closer with that lens)