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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?

mattdm
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DarenW
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3 Answers3

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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.
Jerry Coffin
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    The donut bokeh can be really cool with the right shot:

    http://abbazz.zenfolio.com/img/v1/p293409921.jpg

     – eruditass Aug 02 '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 03 '10 at 04:45
  • @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 03 '10 at 05:06
  • Positive points for mirror lenses are reduced weight and length. This Samyang 800mm lens is only 114mm long: http://www.samyang.pl/product,144,category,5,samyang_800mm_mc_if_f8_mirror – Marc Aug 03 '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 08:15
  • @ysap: In theory you could, but I'm not aware of anything currently available that actually does so. At least as far as I know, your only option would be to build your own (and I'm pretty sure it would be quite non-trivial). – Jerry Coffin Dec 10 '19 at 21:42
  • "The primary reason to use a mirror in the first place is for really large apertures" - isn't the primary reason to use a mirror its lack of chromatic aberration? – Vikki Feb 26 '22 at 15:51
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    @Vikki: I don't think so, at least not for most people most of the time. Most use Catadioptric designs that have chromatic aberration anyway. Unless you're using something like a Newtonian telescope that really has no lenses in the light path, you still get chromatic aberration. Now, it is true that using fewer lenses might reduce chromatic aberration, but honestly I'm not sure that's really a serious issue for most people most of the time anyway. – Jerry Coffin Feb 26 '22 at 17:22
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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.

Johannes Setiabudi
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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)

stevenvh
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