Close-up lens
[Image:Lens filter set.jpg|thumb|Set of three close-up lenses]
In photography, a close-up lens is a simple secondary lens used to enable macro photography without requiring a specialised primary lens. They work like reading glasses, allowing a primary lens to focus more closely. Bringing the focus closer allows the photographer more possibilities.
Close-up lenses typically mount on the filter thread of the primary lens, and are often manufactured and sold by suppliers of photographic filters. Nonetheless, they are lenses and not filters. Some manufacturers refer to their close-up lenses as diopters, after the unit of measurement of their optical power.
Close-up lenses do not affect exposure, unlike extension tubes, which also can be used for macro photography with a non-macro lens.
Optical power
Close-up lenses are often specified by their optical power in diopters, the reciprocal of the focal length in meters. For a close-up lens, the diopter value is positive: the bigger the number, the greater the effective magnification.Higher quality achromatic lenses commonly lack a strength specification in diopters. It can be inferred as the reciprocal of the maximum specified working distance 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. For example, 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.
Working distances and magnifications
Close-up lenses change both the maximum and minimum focus distances of a lens. The range can be rather small.Working at maximum distance
Adding a close-up lens to a lens focused to infinity changes the focus point to the focal length of the close-up lens, that is, the inverse of its optical power. This is the combination's maximal working distance:That distance is sometimes given on the filter in millimeters. A +3 close-up lens has a maximal working distance of.
The magnification is the focal distance of the objective lens divided by the focal distance of the close-up lens; i.e., the focal distance of the objective lens multiplied by the diopter value of the close-up lens:
In the example above, if the lens has a focal distance, the magnification is.
Given the small size of most sensors a insect will almost fill the frame at this magnification. Using a zoom lens makes it easy to frame the subject as desired.
Working at minimal distance
When you add a close-up lens to a camera which is focusing at the shortest distance at which the objective lens can focus, the focus will move to a distance which is given by following formula:being the shortest distance at which the objective lens can focus, and being the diopter value of the close-up lens. This is the minimal working distance at which you will be able to take a picture with the close-up lens.
For example, a lens that can focus at combined with a +3 diopter close-up lens will give a closest working distance of.
The magnification reached in those conditions is given by following formula:
being the magnification at distance without the close-up lens.
In the example above, the gain of magnification at will be.
While it would seem obvious that at this distance you will get the highest magnification, focus breathing can cause more of a difference in actual magnification than the small overall in-focus working distance range particularly for higher strength diopters.