Mirror lens lens
A mirror lens lens or catadioptric system is a special lens whose optical elements are a combination of both mirrors and lenses. It is used in photography as a telephoto lens with a fixed focal length or as a lens with a variable focal length.
In terms of the term, catadioptric means that a mirror has an effect, and dioptric means that a lens that is shone through has a refractive effect. The latter term goes back to Johannes Kepler .
construction
Mirror lens lenses, also called catadiopters, are derived from astronomical telescopes . Due to a beam path bent twice by (in the majority of cases) spherical mirrors, the overall length of mirror lens objectives is typically only about one third to one fifth of their effective focal length . The image errors caused by the mirrors are corrected by the lenses arranged in the beam path (image field flattening system). There are many variants with regard to the internal structure. In addition to comparatively simple systems (see graphic), there are complex systems according to Richter / Slevogt that were manufactured by Zeiss as “ Prakticar 5,6 / 1000 mm” or “ Mirotar ”.
The kinked beam path means that - if a worm gear is used for focusing - the front part of the lens only needs to be moved a little when focusing. The overall length and center of gravity change only slightly. In the case of large mirror lens lenses, bellows extensions attached to the rear end have proven their worth, as some of these optics weigh well over 10 kg and the tube cannot be moved .
Another feature of typical mirror lens lenses is the fixed f-number , which is usually 1: 5.6 or 1: 8.0 (in individual cases 1: 4 or 1: 4.5). This is due to the fact that an iris diaphragm can only be integrated with great effort in this design . As a result, no dimming is possible, so that a smaller at high luminance sensitivity (ISO), a lower exposure time or neutral gray filter ( neutral density filters are used) must. The latter are usually included in the construction of the lens and are replaced by a clear filter disc when not in use.
When specifying the aperture, it should be noted that, as with all lenses, they only refer to the aperture ratio, i.e. the losses caused by the lenses and mirrors are not taken into account. The obstruction, i.e. the shadowing caused by the opposing mirror lying in the beam path, is included in higher-quality systems. With up to half an aperture stop, the reflection losses on the mirror surfaces can be considerably higher than the transmission and reflection losses on objectives that only work with lenses.
The basic structure of the mirror lens differs only slightly from the structure of an astronomical telescope according to Maksutov or a Schmidt - Cassegrain . However, the design requirements are usually different. In the case of catadioptric photographic lenses, the most important thing is to define the image format as accurately and sharply as possible. Furthermore, the evenness ( planarity ) of the image plane is particularly important. Most of these lenses therefore have an image leveling group in addition to the actual spherical mirror optics and the typical correction group that eliminates the errors of the spherical mirrors. This ensures that the focal plane coincides with the plane recording plane of the film or image sensor.
Many astronomical telescopes, on the other hand, are primarily designed for visual observation, i.e. for direct viewing of the image through an eyepiece . Systems specially designed for astrophotography were often referred to as " astrographs " or "astro cameras" (e.g. the Schmidt camera ). Today, such systems are offered as “Fotomaks”, for example. These telescopes are then also specially optimized for a flat field of view and mostly prepared for recording on an astronomical mount .
For optics that are designed visually, there are separate image field correctors , also known as “flatteners”, which, as optional assemblies, ensure the required image field flattening . Due to the current trend towards ever larger image sensors in digital cameras or special astronomical CCD systems, perfect image field flattening is of great importance. The line resolution achieved is also with sensor formats close to the 35mm format well above the line resolution of average negative or slide films.
Due to their design, mirror lens lenses are sensitive to stray light . The use of a lens hood that is well matched to the optics is of considerable importance for optimal contrast reproduction.
Imaging properties
A special feature of the optical imaging of mirror lens lenses are the so-called "blurring rings". This is not a design flaw , but an inevitable feature of lenses of this type: While pure lens lenses light reflections blurred depicted as circles of confusion are played, to show them in mirror-lens objectives in the typical blur rings through the beam path obstructing reversing mirror (secondary mirror). This creates the shape of the light reflections reminiscent of a hole reinforcement ring or donut .
An optical advantage of the mirror lens objective is that there is little or no chromatic aberration . This is due to the fact that the existing lens systems are mostly only weakly refractive and therefore the dispersion no longer introduces any significant impairment.
Areas of application
photography
Mirror lens lenses for 35mm cameras were especially popular in the 1980s, often as f / 8 with a 500mm focal length. This gives photographers a comparatively light and easily transportable telephoto lens.
Particularities:
- The depth of field cannot be influenced by stopping down, except for so-called crooked mirrors . Due to the design, it is determined by the fixed ratio of aperture to focal length and the image scale.
- The blur rings , the special shape of the bokeh of this optics, can appear inappropriately dominant to the main subject. They almost always occur in the out of focus area of this telephoto lens at highlight lights and must be taken into account when composing the image.
Even today, mirror telephoto lenses are offered as new goods and new, manual versions of them appear regularly. A special feature are some mirror lenses from Minolta (produced by Sony after taking over the photo division until 2006), which are the only models with an autofocus.
Night vision technology
Catadioptric systems have also become established in night vision technology, mostly in connection with light amplifier tubes. These special systems with aperture ratios significantly better than 1: 4 are likely to be the most powerful catadioptric systems currently produced.
Amateur astronomy and astrophotography
Since the light intensity is no longer the determining factor of photo optics in amateur astronomy due to the increasingly sensitive digital image sensors and cameras , interesting recordings can be found again and again in the focal length range up to approx. When recording ideal point sources, as they predominantly occur in stellar photography, the quality of the optics ( definition brightness ) and the diffraction as a limiting physical effect determine the recording. The opening is crucial for the ability to collect light . Since the recordings in astrophotography mostly have to be tracked and therefore have to be carried out on special mounts , considerations such as the exposure time play a different role than during the day, when the exposure time decides about the need to use a tripod.
The terrestrial telephoto lenses “MTO 10/1000” from the CIS and the mirror lens lenses known as “Rubinar” (10/1000, 5.6 / 500 and 4.5 / 300) are to be regarded as classics of amateur astronomy . Thanks to their low price and their ease of modification, they are used as small telescopes , guide scopes and photo optics. In the photography of stellar objects, you can fully exploit the color purity (no chromatic aberration ) and the good definition of the images thanks to the flat image field. In particular, users of a comparatively small and light mount can easily set up an additional guide telescope or photo optics without mechanically overloading the mount thanks to the compact design of the catadioptric systems. The short construction and the mostly low weight in relation to the opening facilitate the assembly-friendly configuration.
literature
- Ernst A. Weber: Photo internship. 3rd, revised and expanded edition. Birkhäuser, Basel et al. 1997, ISBN 3-7643-5677-4 .
- Uwe Laux: Astro optics. 2nd, updated and expanded edition. Verlag Sterne und Weltraum, Heidelberg 1999, ISBN 3-8274-1305-2 .
