Why do photographic mirror lenses use Mangin mirrors instead of first-surface mirrors?
Asked 8/27/2013
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In a typical household mirror, the reflective coating is behind the glass, while a first-surface mirror has the reflective coating on the front. Since front-surface mirrors avoid light passing through glass before reflection, it seems like they would be better for catadioptric (mirror) telephoto lenses. However, many photographic mirror lenses appear to use a rear-coated glass mirror element, such as a Mangin mirror. Why is that design used instead of a first-surface metal mirror?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Cost and ease of design.
The main purpose of mirror telephoto lenses is to make them much more cheaply and compact than a conventional refractive lens can be produced. This is easiest to accomplish by coating the back side of a negative meniscus lens (known as a Mangin mirror) instead of creating a parabolic first surface mirror that would need to be shaped and coated in a way that is more expensive to produce and would also require correction for spherical aberration. By coating the back of the meniscus lens the light passing twice through the same lens is corrected by the glass of the mirror itself.
Mangin mirrors were first created for use as catadioptric reflectors in searchlights. They yield better performance and and are less expensive to make than the front coated parabolic reflectors they replaced.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
12y ago
0
Generated from our catalog & community — verify before relying on it.
Photographic mirror lenses often use a Mangin mirror—a negative meniscus lens with the reflective coating on the rear—because it is a practical optical design, not just a cheaper substitute.
A first-surface mirror would need to be shaped very accurately, typically as a parabolic surface, which is more complex and expensive to manufacture. It would also need additional correction for spherical aberration.
A Mangin mirror helps solve both issues at once: light passes through the glass, reflects off the rear coating, and passes back through the glass again. That double pass through the lens element helps correct aberrations, so the mirror itself contributes optical correction rather than just reflection.
So the reason is mainly design efficiency and cost: a rear-coated Mangin mirror can deliver good performance in a compact catadioptric lens while being easier and less expensive to produce than a precision first-surface parabolic mirror.
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