A macro lens is a lens optimized for shooting at distances much closer than normal lenses. Another feature of a true macro lens is the ability to provide the magnification ratio of at least 1:1 at the closest focusing distance. The magnification ratio of 1:1 means that the image of the subject formed on a camera sensor is of the same size as the subject in the real life. Almost all macro lenses with focal lengths of 90-180mm (except for some older designs) are capable of “life-size” magnification. Macro lenses with focal lengths of 35-70mm typically offer the magnification ratio of 1:2. “Life-size” magnification, however, can be achieved by the means of dedicated extension rings or close-up lenses.
The image quality of almost all macro lenses tends to be better than that of standard prime lenses, especially at close distances. Models incorporating a floating elements are also capable of providing consistently high image quality at the entire range of focusing distances from infinity down to the closest focusing distance.
Traditional lens designs, especially older models of macro lenses, move all optical elements as a whole while focusing: the lens barrel extends a considerable distance when the lens is focused at the closest distance. For example, if you need to focus a 90 mm macro lens to 1:1 magnification, the lens has to be extended an additional 90 mm from its position at infinity. This could result in the lens casting a shadow on the subject or even striking it. That’s why older macro lenses tend to focus no closer than 1:2.
Modern designs use internal or rear focusing, in which only the group of elements in front or behind the diaphragm moves during focusing. The overall length of a lens remains constant and the filter thread does not rotate during focusing. Internal focusing also contributes to fast and efficient autofocus. At the same time, internal or rear focusing and floating elements alter the focal length, decreasing it as focusing distance is reduced. For example, the effective focal length of Canon EF 100mm F/2.8 Macro USM is closer to 73mm when focused at the minimum focusing distance of 0.31m. Since the focal length determines the angle of view of the lens, sometimes you will need to reframe your subject after focusing.
Macro lenses with short focal lengths (50-60mm) are cheap, compact and lightweight. However, the closest working distance of such lenses (a distance from the front of the lens to the subject) is too small for shooting insects, as the lens barrel is located too much close to the subject and can cast a shadow on it or even strike it. Macro lenses with long focal lengths (150-180mm) are more expensive, bulky and heavier, but offer larger working distance. This gives you more room to position lighting and also reduces the chances of casting a shadow on a subject. But the most popular true macro lenses are those with medium focal lengths (90-105mm), because they provide a good compromise between various lens characteristics and price.
It should be mentioned that most modern macro lenses offer autofocus, but due to a very shallow depth of field provided by every macro lens even at or near the minimum aperture, manual focusing is a way more reliable method of obtaining perfect focus.