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