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Film - Digital

What is a mirror lens?

As the name implies mirror lenses are mainly based on mirrors rather than glass elements. Incoming light gets reflected by a main mirror (located at the back of the lens) towards a small secondary mirror (at the front) which then reflects the light back towards the film via a correction (glass) element. The lack of glass elements (apart from the correction element) is a significant design advantage compared to classic (refractive) lenses - chromatic aberrations (visible as color shadows) are virtually absent. As the light enters the lens the light path gets folded. This is the reason why these lenses offer such a long focal length while keeping a very physical small length. Dependent on the configuration the main mirror can be fairly large so while quite physically short most mirrors tend to be rather fat as well.


A 500mm f8 mirror lens remain pretty compact but as the speed of the lens is increased to say 500mm f 5.6 or 1000 f11 the mirrors become as large in diameter as the width of an SLR camera.

The schema of a Schmidt-Cassegrain-type mirror lens means that there are some significant disadvantages-

the mirrors reflections tend to eat contrast so they don not perform well with low contrast subjects

the overall sharpness of the lens is usually mediocre

There is no aperture so it's not possible to control the depth-of-field - this is often done by inserting or rotating a series of neutral density filters.

the secondary mirror produces an odd doughnut-like effect on the out-of-focus high-lights.


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