Lenses which feature a reproduction ratio of 1:1, and which are capable of producing an image on the film or sensor the same size as the subject, are described as "macro". However, they tend to be difficult and expensive to manufacture. The lens manufacturers therefore softened this strict definition of the term and applied it to lenses featuring generally high reproduction ratios.
Macro lenses of any description are designed to allow the lens to move further than normal from the image plane so that the image produced on the film or sensor is relatively large. They can be used close to subjects and consequently achieve high reproduction ratios. Although the definition of the term "macro" varies somewhat, it is now commonly applied to lenses featuring a reproduction ratio of at least 1:4. At this ratio, the images produced on the film or sensor would, at maximum magnification, be one quarter of the size of the subject photographed. When an image of this magnification is printed at a size of 6" x 4" (four times the linear dimensions of a 35mm frame - 36mm x 24mm, or 1" x 1.5"), the subject will be reproduced at life size. Some manufacturers use the term "micro lens" to describe lenses of broadly this specification.
Lenses of this type are convenient and relatively compact, but cannot match the reproduction ratios achieved by using extension rings or bellows. They are available in various focal lengths, mostly in the 50mm to 200mm range, and tend to have maximum apertures somewhat smaller than their conventional counterparts.
Using a macro lens requires a certain amount of discipline. When working very close to a subject it is important to hold the camera and lens absolutely steady. Depth of field is minimal and may be only a few millimetres. Apertures of f/32 and f/64 are therefore commonly employed. However, small apertures imply long exposures, perhaps measured in seconds. Hand held work is therefore impossible and a very stable tripod becomes essential. Any camera movement will result in significant blurring of the image when working very close to a subject.