About Macro Lens

Although this is a bit too early to go into details of lens construction, one of our classmates asked this question in class. I think I will try to explain it with the help of some reference on the web.

Marco lens is the special kind of lens that let you get very close to the subject (object), usually within inches distance, to take photos of it. Technically, the resulting image recorded on the sensor should be equal to or even bigger than the original subject (object). To give an example, a macro lens will be able to get you so close to an ant that your resulting image recorded on the sensor is, say, as big as a fly. We call this 1:1 or 2:1 or 4:1 ratio, which means that your resulting image on the sensor is the same size, or double, or 4th times bigger than the original object.
There is also something call Close-up lens which also allows us to get close to a subject to record an image. However, close-up lens cannot get an image that is close to 1:1 ratio on the sensor. A lot of our general-use zoom lens that come with the camera can do a good degree of Close-up lens do, but they should never be called marco lens.

I would say the one on the right is classified as taken by Close-up lens. The one on the left is a photo taken by Macro lens. And I will let you guys argue about the middle one!

The following is what I read on dpreview.com (http://www.dpreview.com/glossary/optical/macro)

In strict photographic terms, "macro" means the optical ability to produce a 1:1 or higher magnification of an object on the film or sensor. For instance if you photograph a flower with an actual diagonal of 21.6 mm so that it fills the 35mm film frame (43.3mm diagonal), the flower gets magnified with a ratio of 43.3 to 21.6 or 2:1, or with a magnification of 2X. Macro photography typically deals with magnifications between 1:1 and 50:1 (1X to 50X), while close up photography ranges from 1:1 to 1:10 (1X to 1/10X).