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In astronomy, telescopes are most commonly built with mirrors, but there are refractors using lenses. All cameras I know of for Earthly purposes — portraits, landscapes, etc. — use only lenses, not mirrors. Are they some special purpose cameras based on mirrors, other than for astrophotography?

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3 Answers 3

up vote 13 down vote accepted

There are mirrors available for most SLR cameras, but their limitations make them fairly special purpose instruments.

  1. Most use catadioptric mirrors, which have a central obstruction that limits the minimum focal length that can be used -- it would be very difficult to keep the central obstruction small enough for the focal lengths most photographers use most of the time.
  2. The central obstruction leads to out of focus highlights being "donut" shaped, which is often deemed unattractive.
  3. A camera lens normally needs an adjustable aperture, which is relatively difficult with a mirror.
  4. mirrors typically give relatively low contrast compared to lenses.
  5. The primary reason to use a mirror in the first place is for really large apertures; you can support the back of a large mirror, where a lens can only be supported at the edges. Almost no camera lens is large enough for this to really become an issue -- for example, a 600mm f/4 still only has a ~150 mm (6 inch) aperture.

Edit: @Marc raises a good point: to be at all fair, I should probably point out some of the strengths of mirrors:

  1. Catadioptrics are usually quite short for focal length (thanks to folded light path).
  2. Usually quite light
  3. Often Inexpensive (especially used -- and they're often barely used).
  4. A pure mirror (with no transmission through glass) eliminates chromatic aberration.
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3  
The donut bokeh can be really cool with the right shot: abbazz.zenfolio.com/img/v1/p293409921.jpg –  Eruditass Aug 2 '10 at 21:09
    
You could make an off-axis parabolic mirror with no central obstruction - also catadioptric lenses have power in both the mirror and lenses –  Martin Beckett Aug 3 '10 at 4:45
    
@mgb: while marginally practical for direct-observation, an off-axis mirror system seems nearly untenable for photography (instead of shooting forward, you'd basically end up shooting back over your shoulder. Yes, cats use both mirrors and lenses (which has one major consequence: you still get some chromatic aberration, which a purely mirror-based system eliminates. –  Jerry Coffin Aug 3 '10 at 5:06
    
Positive points for mirror lenses are reduced weight and length. This Samyang 800mm lens is only 114mm long: samyang.pl/product,144,category,5,samyang_800mm_mc_if_f8_mirror –  Marc Aug 3 '10 at 10:57
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@Jerry, why "shoot back over your shoulder"? You could have a secondary mirror, just off-axis to redirect the light in a normal direction. –  ysap Sep 15 '11 at 8:15

There are some mirror based lenses, such as Nikkor 500 f/8 mirror. You can see it here: http://www.kenrockwell.com/nikon/5008.htm.

It does confirm what Jerry wrote, that all mirror lenses are fixed aperture and with high focal length.

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Advantages of mirror: lower weight and shorter length
Disadvantages: horrible bokeh and (much) greater diameter (think filters!)

Only useful (and available?) for long focal distances (400mm and up)

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