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. The term "micro lens" is also used 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. The longer focal lengths are convenient because they allow the lens to be moved further away from the subject and hence provide space to set up lighting between the camera and subject. If it is not possible to achieve the required reproduction ratio with a single macro lens, extension tubes or bellows must be used. Greater reproduction ratio can be achieved without sacrificing working distance by using a multiplier, although not all multipliers will fit macro lenses.
Some macro lenses are optimized to provide their best performance at a reproduction ratio of 1:1. Others can achieve reproduction ratios of up to 5:1, and can be used to bring subjects such as the structure of an insect 's eye, snowflakes and other tiny objects into sharp focus. Photomicrography begins when the reproduction ratio reaches 10:1, or the image is ten times the size of the subject. Expensive equipment is required to achieve magnifications of this order.
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.