Close-up accessory lenses
If you don't have a lens that focuses sufficiently close, you can mount a close-up lens (sometimes called a diopter lens) in front of one of your camera lenses. The magnification that you can attain depends on the strength of the close-up lens(i.e., its focal length) and the focal length of your camera lens. I will not discuss here how to compute the maximum magnification of a specific combination of close-up lens and camera lens, because this involves several factors besides the two focal lengths. As a rule-of-thumb, given a close-up lens of a certain strength, a camera lens with a long focal length will be more strongly affected by it (i.e., it will focus much closer than without the close-up lens) than a camera lens of short focal length. Another important rule is that the shorter the focal length of a lens, the shorter is its working distance at a given magnification. This applies also to the combined focal length of a lens + close-up lens.
Close-up accessory lenses consist of a single element, or sometimes two elements glued together to correct chromatic aberrations. Multi-element accessory lenses are also available for high magnifications.
Close-up lenses are usually mounted in a metal ring similar to that used for filters that screw onto the front of camera lenses, and are attached to a lens in the same way as filters. Close-up lenses, as used in close-up photography, are convergent lenses, i.e., they focus parallel incoming rays of light to a point on the opposite side of the lens, usually some distance from the lens itself.
Cheap close-up lenses are plano-convex or covexo-concave lenses, not unlike those of eyeglasses. They give only mediocre to moderately good results. More expensive close-up lenses are achromatic doublets (i.e., two lenses of different materials cemented together). These lenses, also called achromats, give far better results. Good close-up lenses are also coated with anti-reflection layers on both surfaces. Nikon markets two such lenses, models 5T and 6T. These are often difficult to find, and the Canon 500D is a good alternative. A further advantage is that the Canon close-up lens is manufactured in several sizes.
The choice of a camera lens to combine with a close-up lens depends primarily on which lenses you have available. You should choose a lens of good quality, because adding a close-up lens may magnify the faults of the camera lens. You should also not use a camera lens of short focal length, because achieving a sufficient magnification will involve using a strong close-up lens (strong ones tend to have correspondingly strong optical aberrations) and will give you a very short working distance. Camera lenses with an excessively long focal length should also be avoided, because they tend to have a long minimum focusing distance to start with, and a small range of focusing distances when a close-up lens is added.
The best camera lenses for this purpose are those of fixed focal length (as opposed to zooms, although the latter can also be used). A lens with a focal length of 50 to 135 mm is most practical for this use. The strength of an appropriate close-up lens depends on the magnification to be attained, and on the focal length of the lens being used. Stronger close-up lenses must be used for higher magnifications and lenses of shorter focal lengths.
Close-up lenses can be used also with cameras that don't have interchangeable lenses. In fact, for these cameras they are the only method available for reducing the minimum focusing distance. Most modern point-and-shoot digital cameras can focus to a very close range, and can provide surprisingly good performance in close-up photography. Their main drawback is that the working distance typically is extremely short (frequently, 10 to 40 mm at maximum magnification). This makes it difficult or impossible to place light sources in appropriate positions around the subject (especially, close to the optical axis of the lens). In this situation, close-up lenses cannot help.
A lens is a lens is a lens
There is some confusion in the use of the term lens. Basically, a lens is a sheet of transparent material that changes the direction of light rays passing through it. Typically, one or both surfaces of a lens are curved, but this is not an absolute requirement. There are holographic lenses, for instance, that have flat sides and bend light by passing it through a microscopic, laser-etched pattern embedded in the lens material. Out in the universe, there are gravitational lenses that bend light just by attracting the mass of photons. The lenses used in eyeglasses and cheap magnifiers normally are made of a single optical element (i.e., a single piece of a homogeneous glass or plastic). These can be called simple lenses. The lenses used in cameras, instead, usually are made of a stack of several optical elements. Each of these optical elements is also a lens. Optical elements may be glued together, and therefore a group of these lenses may appear to an observer to be a simple lens (but its optical properties are of course different from those of a simple lens).
Camera lenses are of course very different from a simple, unmounted lens. Camera lenses are made up of a complex set of optical lenses, precision mechanics and electronics, and may contain over a hundred separate parts. It is unfortunate that, in the English language, the term lens is used also for camera lenses. Other languages use distinct words for simple lenses and camera lenses.
To add to the confusion, one sometimes encounters the term lense. It may be used to indicate a camera lens. In spite of what some people may tell you, a lense is exactly the same thing as a lens. The lense spelling is just old-fashioned, like in the expression ye olde shoppe.
Close-up lenses (also called diopters or add-on lenses) are also lenses. In this web site, normally I use the term lens when speaking of camera lenses. In the case of close-up lenses, I call them so to make it clear when I am talking about the camera lens, and when I refer to the close-up lenses.
How many diopters in a diopter?
A further source of confusion is that a diopter, or dioptre, is also a measurement unit that expresses the power of a lens. I am not using these units in my web site, but you may encounter them elsewhere. In practice, for a converging lens the strength in diopters (D) is given by D = 1000 / f, where f is the focal length in mm. therefore, a lens with a focal length of 200 mm has a strength of 5 diopters. Therefore, you may choose to express the strength of a lens either in focal length or in diopters - it is just the same thing expressed with two different measurement units. In this web site, I always use the focal length because this is the measurement normally used for camera lenses. The strength of eyeglass lenses, instead, is normally given in diopters.
Cheap close-up lenses lenses for close-up work often come in sets of three, with strengths of 1, 2 and 3 diopters. Achromatic close-up lenses may have higher strengths (roughly between 5 and 10 diopters). An even higher diopter value requires a lens design consisting of several optical elements, in order to better correct the inevitable optical aberrations.
The strength in diopters can be specified also for diverging lenses, but in this case the value in diopters is negative. This expresses the fact that the focal point of a diverging lens is located on its opposite side, with respect to a converging lens.
When to use close-up lenses
Close-up lenses are likely to work acceptably when used at close-up distances, but reaching the macro range (i.e., 1:1 or higher) with close-up lenses and achieving good quality is difficult. At 1:1, the focal length of a close-up lens must be equal to that of the camera lens, and the optical aberrations introduced by close-up lenses of short focal lengths are conspicuous. In fact, at this magnification it is easier to achieve good results by attaching a reversed camera lens in front of one mounted normally. The added lens will work like a close-up lens, and likely will provide a better correction of aberrations than simple or achromatic close-up lenses. Problems connected with reversed lenses are discussed here.
Some photographers have reported better results with a camera lens and moderately strong close-up lenses of good quality, than with the same camera lens mounted on extension rings. The reason for this is likely that the camera lens used is a general-purpose type not optimized for close-up work. Increasing its distance from the focal plane by using extension rings forces it to work outside the focusing range for which it is optimized, while using a close-up lens does not do so (although it does introduce aberrations produced by the close-up lens). This is less likely to happen with camera lenses designed for macro and close-up photography (albeit, in this case most macro lenses should be able to focus at the desired distance without using extension rings). Special close-up lenses dedicated to (i.e., matched to, or optimized for) a particular model of macro lens, and apparently providing very good results, occasionally have been manufactured. For instance, such a close-up lens was available for the Olympus 80 mm macro lens, and the Zeiss Tessovar uses dedicated close-up lenses mounted in a rotating turret to extend its range of magnifications.
It is also possible to use close-up lenses and extension rings at the same time, although only experimenting can tell you if this provides any practical advantage over using either one alone.
Close-up lenses may be the only way to perform close-up photography with wide-angle lenses. Because of the short focal length of these lenses, it may not be possible to use extension rings. For instance, a 10 mm super-wide-angle requires an extension ring of about 3 mm to achieve a reproduction ratio of about 1:3. Unfortunately, the shortest extension rings are about 9 to 11 mm thick, which implies a reproduction ratio of about 1:1. At this reproduction ratio, the subject would have to be placed inside the lens to achieve focus. There may be practical problems in using close-up lenses with super-wide-angle lenses. For instance, for a close-up lens to be useful, the focal length of the close-up lens must be of the same order of magnitude (usually, 2-5 times longer) than the focal length of the camera lens. The close-up lens must also be large enough to mount in front of the lens without causing vignetting. A large close-up lens of short focal length (assuming one can find such a lens) may introduce strong aberrations, thus making it useless, except as a novelty for special effects.