Close-up filter


Close-up filter
Set of three close-up lenses
Typical close-up lens
Optical scheme of close-up photography.
1 - Close-up lens.
2 - Camera objective lens (set to infinity).
3 Camera.
4 - Film or CCD plane.
y - Object
y" - Image

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.

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. A split diopter has just a semicircular half of a close-up lens in a normal filter holder. It can be used to photograph a close object and a much more distant background, with everything in sharp focus; with any non-split lens the depth of field would be far too shallow.

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