All modern, general-purpose macro lenses can focus between infinity and 1:1. Older macro lenses needed a matched extension ring in order to achieve 1:1, and typically attained only 1:2 without the ring. Most modern macro lenses focus from infinity to 1:1 without an extension ring. These lenses have a fixed focal length (albeit, see below) that ranges between 50 and 200 mm. From a mechanical point of view, these lenses are built essentially like ordinary lenses, but have longer helicoids and/or moving internal groups of optical elements to allow the extended focusing range. They also tend to have a higher sharpness and correction of optical aberrations than non-macro lenses. All major camera and lens manufacturers currently produce one or more models of macro lenses of this type.
Specialist macro lenses typically cannot focus at infinity, and most cannot be used at all without additional equipment. This is the case, for instance, of the Olympus 135 mm f/4.5 shown above. To use them, you must mount them on bellows or extension tubes (see here). Some of these lenses provide reproduction ratios as high as 20:1, while others are optimised for reproduction ratios ranging to 1:1 or less. Their focal lengths are much variable, and range from roughly 10 to over 200 mm. The shorter focal lengths are used for higher magnification. There are several classical models of specialist macro lenses (e.g., the Zeiss Luminar series) which still command high prices on the second-hand market. Very few, if any, of these lenses are currently produced.
The large majority of macro lenses is of very good to extremely good optical quality. The principal exceptions are among the legacy Soviet macro lenses. The mechanical quality of most macro lenses also ranges from adequate to very good. As a consequence, macro lenses are more expensive than general-purpose lenses of comparable focal length.
Older macro lenses focus by moving back and forth as a whole within their focusing helicoids. Specialist macro lenses work in the same way when mounted on bellows. Modern macro lenses, instead, often have a fixed group of optical elements at their back, or one or more groups of elements that move on separate cams from the rest of the lens. These are called floating groups. This design allows the lens to be corrected for aberrations at an extended range of focusing distances.
Some macro lenses (especially those of higher focal lengths) possess internal focusing (i.e., their front and back elements remain immobile when focusing, and only internal elements move). Among the macro lenses shown here, the two at the left and centre possess a fixed correcting group, the one at the right internal focusing. The figures show the lenses focused at infinity (top) and 1:1 (bottom). The two lenses at the left and centre possess a complex system of concentric barrels that allow the lens groups to move a much larger distance than suggested by the increase in length of the outer barrel when focusing.
The use of fixed, or independently moving groups of elements results in a change in the focal length of the lens as it focuses. This is roughly the same principle used to build zooms, but in the latter the focal length can be changed independently of the focus setting, while in macro lenses the two change together. Changing the focal length allows manufacturers to better control the optical properties of a macro lens, and to extend its focusing range without using heavy and expensive mechanical parts. For instance, a 200 mm macro lens that focuses by moving its elements back and forth in a single group would need to extend forward by 200 mm to achieve a 1:1 magnification. This would result in the length of the lens more than doubling when extended, and would require a large, heavy and expensive multi-stage focusing helicoid. By reducing the focal length to about 100 mm (with the use of floating elements) when focused at 1:1, instead, the lens does not need to extend at all, and can have internal focusing. This makes the lens lighter and more resistant to dust (an extending lens sucks in air, and dust with it) and mechanical abuse (the only moving external part is the focus ring).
The principal drawback of a shortened focal length at close range is that the working distance decreases accordingly. This is especially true of macro lenses of high focal length, because the savings achieved by manufacturers in this way are proportionately higher. On the other hand, the main reason for choosing a macro lens of high focal length is precisely its higher working distance, so a macro lens with internal focusing may fall short of your expectations in this respect. For instance, the Sigma Apo Macro 180 mm EX (rightmost in the above figures) is roughly a 185 mm at infinity, but becomes a 98 mm at 1:1. This lens is exceptionally good in many respects, but not if you expect exactly the working distance theoretically associated with a 180 mm. The amount of change in effective focal length of macro lenses is poorly documented by their manufacturers. Information on popular brands and models provided by users is often found by making a web search.
Macro lenses and "macro" zooms
Lenses used in macro photography must be able to provide a reproduction ratio of at least 1:1. Anything less than this qualifies, at best, as close-up. However, for the last three decades, many consumer-grade zooms have been called "macro" by their marketers. Almost invariably, these zooms stop significantly short of reaching a 1:1 reproduction ratio.
The marketing stratagem of marking consumer zooms as "macro" lenses originated as an attempt to cram more functions into a lens, in order to make it desirable to a larger group of consumers. It was cheap and relatively easy to extend the focusing and zooming cams of a zoom to allow close-up focusing. Typically, though, close-up focusing becomes possible only at a fixed focal length, or within a narrow range of focal lengths. The quality of pictures taken at the close-up setting with the first "macro" zooms was mediocre. This has improved, but these zooms are still far from being as good as dedicated macro lenses. Resolution is better than in the first examples of "macro" zooms, but curvature of field and distortion, with very few exceptions, still are major problems. In addition, "macro" zooms typically achieve a reproduction ratio of only 1:3 to 1:6. The two examples shown above (both at the minimum and maximum extensions of their helicoids) do so, and perform poorly at close-up distances, when compared to true macro lenses.
The improper use of the term "macro" creates confusion among unprepared buyers. As an example, Sigma is offering several "macro" consumer-grade zooms that provide a 1:3 to 1:3.5 reproduction ratio at a quality that does not satisfy exacting requirements, alongside with a line of true macro lenses that provide 1:1 and excellent performance (for a much higher price). Sigma indiscriminately calls both types of lenses macro. The maximum reproduction ratio of true macro lenses is listed in their technical specifications, but often this crucial bit of data is missing from the blurb that pushes "macro" zooms.
There are very few, if any, true macro zooms. One of them is the Micro Nikkor AF 70-180 f/4.5-5.6, which reaches 1:1.3. It is not a true macro in reproduction ratio, but it is sufficiently close, and its optical performance is very good. This lens is discontinued. Apparently, it was too expensive to produce, and there is not enough demand for it. After all, in close-up and macro photography it is customary to change the focus settings and to move the camera back and forth in order to change magnification, so the real usefulness of a macro zoom is questionable. In some situations, it may be more comfortable to use than a lens of fixed focal length (for instance, a zoom allows changes of reproduction ratio without having to move the camera relative to the subject). However, there is nothing that a macro zoom can do, that cannot be duplicated (with a little extra work) by using one or more macro lenses of fixed focal length.
There are only three zoom specialist macro lenses that I am aware of. One of these is the Zeiss Luminar Zoom (apparently optimized for the 2.5x-5x magnification range). The two others are the Zeiss Tessovar and the Wild line of macroscopes.
The "macro" label has been applied also to a few telephoto lenses. Also in these cases, the lenses do not have true macro capabilities, but only close-up. In some cases, like the 600 mm f/8 mirror lens shown here (which focuses between infinity and 1:3) the depth of field is too shallow for the close-up capabilities to be really useful, and the picture quality at close-up range is only mediocre. Nonetheless, the working distance (about 1.8 m at 1:3, i.e., a 72 by 48 mm subject area with a Nikon DSLR) is unbeatable if you cannot get any closer to the subject.
Alternatives to macro lenses
An alternative to a macro lens is provided by the use of close-up lenses mounted in front of a general-purpose lens. The best lenses for this purpose are those of fixed focal length, as opposed to zooms. A focal length of 50 to 135 mm is most practical for this use. The strength of an appropriate diopter depends on the magnification to be attained, and on the focal length of the lens being used. Stronger diopters must be used for higher magnifications and lenses of shorter focal lengths.
A reversed camera lens can be used instead of a close-up lens.
Another alternative involves the use of a teleconverter between lens and camera body. A 2x teleconverter added to a lens capable of 1:1 magnification will effectively bring it to 2:1. The interesting fact is that the working distance (i.e., the distance between the front of the lens and the subject) will not decrease, so using a teleconverter may help you to photograph animals that are easily frightened when approached. The loss of light and the overall decrease in sharpness associated with the use of teleconverters also applies to macro photography.
Extension rings and bellows can be used with non-macro lenses to reach the close-up and macro ranges. In this case, when the magnification approaches or exceeds 1:2-1:1, the lens probably will perform better if reversed.
Enlarger lenses are optically corrected for short working distances, and therefore can be used in close-up and macro photography. They may need to be reversed for best performance (especially in photomacrography), their diaphragm must be operated manually, and they must be mounted on bellows or extension rings because they have no focusing helicoids. In these respects, enlarger lenses are comparable to specialty macro lenses, but usually are less expensive, and an alternative worth trying for high-magnification macro photography in studio settings. While the performance of the cheaper enlarger lenses is likely to be disappointing, good results are often reported with high quality ones, like the EL-Nikkor series and the Leitz and Zeiss brands.