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A coupling ring is used to establish a connection between two lenses. In front of the actual lens, a second lens is mounted in the inverted position. Both lenses are connected via their filter threads: The first lens is mounted in the normal position on the bayonet of the camera. The coupling ring is then screwed to the end of the camera lens. This has a threaded ring with two external threads. At the other end another lens is screwed in retro position. Coupling rings can also be used with compact or bridge cameras if they have an appropriate filter thread.
The front lenses of both lenses are directed towards each other, whereby the second lens functions as a close-up lens. Retro lenses have superior optical performance compared to close-up lenses. This allows high image scales to be achieved with very good image quality. Coupling rings are therefore used primarily in macro photography as a cheaper alternative to a macro lens. For optimum results, the aperture of the front lens should always be wide open. The depth of field is normally regulated by the camera lens.
You can calculate the possible magnification by dividing the focal length of the camera lens by the focal length of the attachment lens. You can achieve a strong magnification by using a long focal length lens on the camera and coupling a short focal length lens to it: If you connect a 100 mm portrait lens with a 25 mm wide-angle lens using a coupling ring, you get a magnification scale of 4:1. The highest magnification factor can therefore be achieved with a combination of telephoto lens and wide-angle lens.
Not every lens combination works equally well: wide angle lenses or normal focal lengths with large apertures are best suited for use in retro position. If the aperture of the front lens is too small, the entire image format may not be available. In this case, clear vignetting may also occur.