Close-up lenses or diopters mount on the end of your lens like a filter would, but have a curved surface to produce a magnification of the image. They are preferred by some over extension tubes because they are the only way to add magnification without a loss of light. However many of the single element close-up lenses can degrade the image. The dual element lenses perform significantly better.
Close-up lenses are specified according to their power in diopters, which commonly range from .25 to 10. You can stack two lenses together as long as the more powerful one is placed closer to the lens. The Canon D(ouble) and the Nikon T(wo) Close-up lens are double element lenses that produce fine images.
The Canon 250D (+4) and 500D (+2) are widely used and are not Canon specific. The 250D is made for focal lengths from 30 – 135mm, while the 500D is for focal lengths from 70 – 200mm. One reason the Canon lenses are so popular is that they come in a variety of filter sizes. The Nikon 3T (+1.5) and 4T (+2.9) come in a 52mm filter size while the 5T (+1.5) and 6T (+2.9) are made in a 62mm filter size.
Continue with next section: Stacking Lenses