Design. Long Focus Lenses in Digital Cameras

In principle, a lens of greater focal to normal is a group of converging lenses which, when focused on infinity, is situated at a distance from the film place equal to the focal length. In a plate camera, the lens is place on the extreme, and who's extension depends on the focal length and the focus distance. In a lens with a focus mechanism, the lenses are mounted on a metal cylindrical carcass, whose dimension and weight depend on the focal. The only converging element forces us to place the group in a hollow tube of greater length the longer the focal is. In practice, in most "long" lenses, a space economizer design is adopted.

•  Telephoto: In a strict sense, the word "telephoto" refers uniquely to a design were the physical length of the lens is inferior to the focal. Instead of using a single converging element, placed at a distance from the film equal to the focal length (focus on infinity), a group of smaller focal lengths than the total can be combined and placed closer to the film, who's most convergence is reduced by a posterior divergent group.

•  Catadioptric Lenses: The problem of size and weight of the super telephotos means a limitation of it's manageability. Many manufacturers have resolved this problem going to the catadioptric design which, schematically, consists on folding the beam of light in three with the use of concave mirrors and lenses. The light travels a route towards the camera when going in the front end of a lens ( collector ), is reflected forwards by a concave mirror placed around the mount, and is reflected again towards the viewfinder or the film by another mirror placed in the center of the internal face of the collector.

This design spectacularly reduces the size and weight of telephoto lenses but demands a suppression of the diaphragm. The formation of images by reflection is free of some of the characteristic aberrations of lenses and refraction . A catadioptric lens, besides it's apparent simplicity and lack of a variable aperture, provides images of a equal sharpness or better than a telephoto of the same focal length. The depth of field at each distance is impossible to alter, and the intensity of light transmitted can vary only with the use of filters.

A peculiar characteristic of this type of lens is that the confusion circles correspondent to out of focus unfocused points are reproduced not as discs or polygons (the shape of the aperture of the diaphragm) but as rings (because of the shape of the front part, through whose center no light passes).