Close-up lens
In photography, a close-up filter, close-up lens or macro filter is a simple secondary lens used to enable macro photography without requiring a specialised primary lens. They work identically to reading glasses, allowing any primary lens to focus more closely. It is actually more appropriate to use the close-up lens terminology as it is a lens and not a filter, although close-up lenses typically mount on the filter thread of the primary lens, and are manufactured and sold by suppliers of photographic filters. Some manufacturers refer to their close-up lenses as diopters, after the unit of measurement of their optical power.
While some single-element close-up lenses produce images with severe aberrations, there are also high-quality close-up lenses composed as achromatic doublets which are capable of producing excellent images, with fairly low loss of sharpness.
Close-up lenses are usually specified by their optical power, the reciprocal of the focal length in meters. Several close-up lenses may be used in combination; the optical power of the combination is the sum of the optical powers of the component lenses; a set of lenses of +1, +2, and +4 diopters can be combined to provide a range from +1 to +7 in steps of 1.
When you add a close-up lens to a camera which is focusing to infinity, and you don't change the focus adjustment, the focus will move to a distance which is equal to the focal length of the close-up lens. This is the maximal working distance at which you will be able to take a picture with the close-up lens. It suffices to divide 1 by D, the diopter value of the close-up lens, to get this maximal working distance in meters:
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