# Close-up lens

Close-up lenses are optical lenses that are attached to the lens like filters in order to enable macro photography with normal lenses . Their effect is comparable to reading glasses. As with these, the power is given in diopters .

## functionality

### Working distance

The maximum working distance of a close-up lens (the distance from the center of the lens to the object) corresponds to its focal length and thus the reciprocal of the refractive power. The refractive power of lenses is generally given in the unit dioptre ( dpt , m −1 ). For a close-up lens with +2 D there is a focal length of 0.5 m, for a lens with +4 D a maximum working distance of 0.25 m. In this position the close-up lens images the object to infinity and can be projected onto the image plane with the normal lens in the focus position infinitely .

### Reproduction scale

As magnification m is the ratio of image size B to object size G :

${\ displaystyle m = {\ frac {B} {G}}}$ If the object and its image are the same size, the image scale m has the value 1.

In order to calculate the image scale for a system consisting of a close-up lens and objective, you can use the image formula:

${\ displaystyle {\ frac {b} {g}} = {\ frac {B} {G}}}$ this ratio can be replaced by that of image distance b to object distance g . If the object to be imaged is in the focal point of the close-up lens (object distance) and the image distance corresponds to the focal length of the lens (see section Working distance ), it follows: ${\ displaystyle f \! \, '}$ ${\ displaystyle f}$ ${\ displaystyle m = {\ frac {f} {f '}}}$ At the maximum working distance, the image scale corresponds to the ratio of focal length (objective): focal length (close-up lens). With smaller working distances, the lens must be focused on smaller distances; this is possible up to its closest focusing limit. Depending on the lens used, even larger image scales can be achieved.

An alternative derivation of how a close-up lens works is possible with the effective focal length. The effective focal length of the lens and close-up lens arrangement follows directly from the lens equation :

${\ displaystyle {\ frac {1} {f _ {\ mathrm {eff}}}} = {\ frac {1} {b}} + {\ frac {1} {g}} = {\ frac {1} { f}} + {\ frac {1} {f '}}}$ The close-up lens shortens the focal length f of the objective while the image distance b remains constant. So that the close-up lens acts as a pull-extension order . However, this consideration only applies if the focusing is carried out by means of a drawer extension (the entire lens package is moved away from the image plane). In the case of lenses with internal focusing , the focus is set by shortening their focal length so that and are no longer constant. ${\ displaystyle (f-f _ {\ mathrm {eff}})}$ ${\ displaystyle f}$ ${\ displaystyle f _ {\ mathrm {eff}}}$ ## Image quality

With good close-up lenses can be for lenses that are designed for infinity, shooting skills than with intermediate rings or bellows . The image quality of a combination of close-up lens and normal lens compared to a macro lens is comparable with a strong stop -down lens , but macro lenses are superior with open apertures.

• The lens used is operated in the focus area near infinity, for which it was designed.
• All of today's automatic functions are retained: aperture control, exposure metering, autofocus , and even image stabilization to a limited extent; Corrections such as B. effective aperture are not necessary.
• A close-up lens can be used with any camera and lens, especially zoom lenses , provided the filter thread fits or can be adapted. In the case of intermediate rings, this depends on the optical design.

• Close-up lenses influence the optical system of the lens, but are usually not adapted to it.
• Since close-up lenses are usually placed in front of the lens, they depend on the connection dimensions of the lens, which is usually the filter diameter.
• Simple close-up lenses show chromatic aberration , so an achromatic lens consisting of two cemented lenses is the better, albeit more expensive, solution.

There are different close-up lenses with different magnifications. They can be combined with each other. If you use several lenses together, the image quality deteriorates.

Mechanical vignetting can occur with close-up lenses with a significantly smaller diameter compared to the largest lens in the objective, because the ring around the small close-up lenses acts like a pinhole in front of the objective.

## Tunable cameras (incomplete)

Few cameras can be adjusted to a close-up lens. Close-up lenses can actually be used without adjustment, but they add one, two or more lenses to the lens. The possibility of coordinating a camera with a close-up lens is rarely discussed in articles on macro photography.