DEFINITION:
Macro lenses let us get much closer to a subject than a macro setting on a zoom lens. The optics in macro lenses are also designed to give superb results when shooting close-up pictures.
Macro lenses give us a dramatic, and sometimes larger than life, view of small subjects. Most true macro lenses focus down to "life-size" magnification. In other words, the image projected onto the film or imaging sensor is exactly the same size as the small subject is in real life.
A 100mm macro lens provides a greater working distance from a subject than a 50mm macro lens. This means we can work further away from a small object and still fill the frame with it when using a longer focal-length macro lens.
With macro lenses, depth-of-field is relatively shallow, so extra carefull focus and the use of a tripod or a ring light is recommended. Many professionals typically shot most of their close-up images at small apertures such as f/16 or f/22, for the greatest sharpness from in fromt of the subject to behind it.
Source: EOS Digital Rebel XT Tutorials - lesson 2
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