Today we will show you the brief introduction of catadiopetrc lenses and decentralized lenses.
The catadioptric lenses
The first catadioptric schemes were created to contain the size of telescopes. The innovation was then imported into the photographic field and thus the famous mirror lenses were born, also called mirror lenses, reflective lenses or reflex lenses.
A catadioptric lens is characterized by the presence of two mirrors within its own scheme. These elements allow you to combine the effect of reflection with that of refraction, so as to reduce the physical length of the lens by even 2/3 compared to refractive-only models of the same focal length.
In a catadioptric scheme, the front lens collects the light rays coming from the subject and transmits them to a concave mirror in the shape of a crown placed on the bottom of the lens; this primary mirror reflects the light towards the front of the optical scheme, where there is a secondary mirror with a convex shape which in turn sends the light beams to the sensor or film, through the opening in the primary mirror. Since only reflected light reaches the focal plane, which by its nature is free from dispersion, reflex reflectors produce images substantially free of chromatic aberrations. On the other hand, however, spherical aberrations remain a problem and are often less correct than in refractive lenses.
A big limitation of reflectors is the reduced brightness, which generally varies between f / 5.6 and f / 11 and which due to the low transmission often results in a T value between 8 and 16. It should also be considered that mirror lenses do not have of diaphragm and therefore their brightness cannot be changed. Another factor to take into consideration is that retro-reflectors are rather delicate. Mirrors dull and misalign more easily than lenses and are more prone to mass expansions and contractions as a result of thermal variations. To all this we must add the fact that mirrors clearly reflect on the images even those small deposits of dust and those light spots that would be irrelevant on a lens.
The distinctive feature of images made with a mirror lens is the donut bokeh, which is the “donut” appearance of out-of-focus specular lights, due to the shape of the primary mirror through which the light reaches the focal plane.
The decentralized lens
According to DZOFilm, the decentralized lenses were born to help photographers avoid perspective alterations in architectural photographs. These objectives are equipped with a circle of coverage well above their target format and with a mechanism that allows them to move their optical center both horizontally and vertically, keeping the optical axis perpendicular to the plane of focus.
Let’s imagine we are shooting the facade of a building. If we tilt the camera upwards, we create an image with falling lines; to avoid this effect we can keep the camera parallel to the building, but in doing so the upper part of the building remains outside the frame. With a decentralized lens it is possible to keep the focal plane parallel to the shooting plane, translate the optical axis of the lens upwards and exploit the lower part of its circle of coverage, so as to include the entire building in the frame, avoiding its perspective deformation.
Some decentralized are also tilting and therefore can tilt their optical axis with respect to the sensor, making the plane of focus transverse to it. In this way it is possible to manage the depth of field in a completely unusual way, for example by focusing on a subject at one meter and one at three meters, leaving the intermediate space out of focus.

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