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I like the bokeh and limitations of mirror lenses. I however, also love my 50mm 1.4. Does a 50mm mirror lens exist? I searched but could not find one. If not 50mm what is the closest to 50mm one can get?

  • can you specify more your camera/system of interest? – aaaaaa Dec 16 '17 at 22:55
  • @aaaaaa just in general. I would prefer cannon at the moment, but I am also looking to buy a new body, so any system is fine – 10 Replies Dec 16 '17 at 22:56
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    Other than for artistic reasons, what would be the point in building one? Come to think of it, fitting the optics into a lens that short would be an interesting exercise, and you certainly wouldn't get f/1.4 out of it. – Blrfl Dec 16 '17 at 23:20
  • @Blrfl not sure.. maybe more light transmission – 10 Replies Dec 16 '17 at 23:21
  • For every mm² you cover in the center of the lens you would need to increase the area of the outer part of the lens by at least the same amount to maintain the same transmission. In practice, due to the depth of pixel wells and their lower sensitivity to angled edge rays compared to more collimated center rays, you would need to increase the outer area by more than you reduce the center area of the entrance pupil. – Michael C Dec 18 '17 at 10:44
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The mirror lens design resolves two key optical problems:

  1. All lenses suffer from chromatic aberration. This is color fringing due to the failure of the lens to refract (bend inward) all colors of light accurately. In a conventional lens, this is accomplished by sandwiching two or more lenses, of different powers using different recipes for the glass. The result is an achromatic (without color error) lens. The mirror lens avoids chromatic aberration because it uses a “first surface” mirror. The mirror coat is thus on the surface of the primary lens. This configuration evades chromatic aberration because the image forming light does not transverse the primary lens element. Chromatic aberration gets worse if the lens is a powerful telephoto. Thus mirror lenses are favored for super long focal length lenses.

  2. The mirror lens folds the light path thus a long focal length can be accommodated by a shorter lens barrel.

The 50mm lens is only about 2 inches in focal length. Correcting chromatic aberration in such a short lens is a piece of cake. Lens designers would never attempt to fold a 50mm and make the barrel super short except for a very specialize application. Bottom line, you are not likely to find a find a 50mm mirror lens.

You like the bokeh! It is created by the fact that the mirror lens uses two mirrors. The secondary is up front. It is a small silvered circular mirror centered in the tubes entrance. You can experiment and obtain nearly the same bokeh with your existing 50mm. Mount a UV filter and glue an obstructing circle on it's center. Cut some opaque disks, different sizes and using double stick tape, and affix one to the center of the UV. Now make a shot or two and then try a different size opaque center blocker. You might like to know, some famous portrait lenses of the past used a center opaque obstacle.

  • I have a problem with your statement about chromatic aberration. My 50mm at 1.4 has ridiculous CA. I can't take pictures of shiny metallic things in any light at f1.4 – 10 Replies Dec 16 '17 at 23:31
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    @ 10 Replies -- Chromatic aberration is mitigated but not eliminated by sandwiching a strong positive power lens and a weak negative power lens together. Both exhibit opposite chromatic aberration so this design mediates but does not eliminate. Highly corrected lens are expensive, you get what you pay for. Try the center obstacle it will deliver bokeh akin to the mirror lens. – Alan Marcus Dec 16 '17 at 23:37
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    I went looking for examples - this is what I found: photocornucopia.com/1053.html – rrauenza Dec 17 '17 at 4:28
  • @ rrauenza --- A tip of the hat from Alan Marcus – Alan Marcus Dec 17 '17 at 6:21
  • @10Replies What you see as CA is probably bleeding. – Michael C Dec 18 '17 at 9:31
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I don't know if anyone still cares about this, but you can get very short focal length mirror lenses. Schwarzschild microscope objectives use mirrors. They might have focal lengths ~0.5-5mm. These are highly corrected for microscopic work, not for conventional photography.

As others have mentioned, mirror lenses have a central obstruction, this would reduce the speed of the lens (light input) and also lower contrast. Also, the sharpest part of a lens is near the optical axis (the centre), with a large central obstruction, this region is lost.

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