Close-up lenses screw to the filter mount of your lens to give increased magnifications. Unlike extension tubes, there is no loss of light. Canon's close-up lenses are doublets (D) – two lens elements cemented together to overcome some of the aberrations of single element lenses. For optical quality, Canon's close-up lenses take some beating.
Canon makes two close-up lenses.
– Type 250D is designed to fit lenses with a focal length from about 35mm to 70mm.
– Type 500D is intended for lenses between 70mm and 300mm focal length.
The magnifying power for a Canon close-up lens is calculated by taking the focal length of the main lens and dividing it by the type number of the close-up lens. For example, a 100mm lens fitted with the 500D close-up lens gives a magnification of 100/500 (so 1/5) or one-fifth life size (0.2x).
If you have several lenses which you'd like to use with a close-up lens, choose the largest filter size available in the strength you want, and step down with a stepping ring.