Close-up lenses don't attach to your camera's lens mount, but instead screw into the filter thread at the front of a camera lens. Because of this, they are sometimes called close-up filters. However, since they do not filter light, this is not strictly correct. You will also see them referred to as supplementary lenses because they are used in addition to another lens.
Whatever you call them, a close-up lens takes up very little room, yet can transform the capabilities of your other lenses, enabling them to focus closer than normal, much as if you attached a magnifying glass. It is an ideal accessory to carry when you want to travel light.
Over the years, Canon has made a range of different close-up lenses with two designs − single element and double element. The double element kind are identified by the letter D. Both kinds have been available in a range of focal lengths including 240mm, 250mm, 450mm and 500mm − this determines the magnification. They also come in different filter thread sizes such as 52mm, 58mm, 72mm and 77mm, but not every permutation of focal length and thread size is available.
The most important factor when choosing a close-up lens is performance, and this depends on the construction. Single element close-up lenses are relatively inexpensive and may be adequate for occasional use, but they are not as good as their double element equivalents. In simple terms, all single element lenses show aberrations, usually chromatic. By adding a second element, the aberrations from the two elements can be made to cancel each other out. The resulting double-element or "doublet" lens may not be completely free of aberrations, but it will usually be a lot better than a single element lens.
The improved performance is particularly noticeable at the edges of the image. This means that if you are photographing a flower, where the subject is mostly in the centre of the frame, a single element close-up lens might be adequate. However, if you are shooting a flat subject that extends to the edges of the frame, such as a postcard or stamps, a doublet lens will give much better sharpness at the edges, especially at wider lens apertures.