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I found this review of the Samyang 300mm f/6.3 mirror lens being sharper in the corners than in the middle. Is that a common characteristic of mirror lenses?

  • Since mirror lenses reflect light, they don't have the aberrations that other lenses do, such as chromatic aberration. Those aberrations contribute to loss of corner sharpness. – xiota Feb 25 at 1:16
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Mirror lenses, as well as catadioptric and Newtonian reflector telescopes, all have an obstruction in the center. This leads to a loss of contrast in the center of the field. Contrast is intimately related to acutance, which many folks call "sharpness."

Even traditional refractive lenses with field curvature can be focused for the edges to be at their sharpest, rather than focused for the center to be at its sharpest. If the field curvature is strong enough, this can give sharper results on the edges than the center of a flat test chart when so focused.

The reduced performance in the center of the field could also be due to using a spherical mirror (cheaper and easier to make) instead of parabolic mirror (more difficult/expensive to make) and focusing optimized for the edges instead of the center when the test images were made.

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This lens is a catadioptric system lens. The central reflector and front lens creates "the annular shape of defocused areas of the image, giving a doughnut-shaped 'iris blur' or bokeh." See the image on Wikipedia. This may be what causes the apparent lack of sharpness on center.

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    "This lens is not a simple reflector with a planar diagonal mirror" – Who said it was? – xiota Feb 25 at 1:11
  • That was my theory as well, I am trying to find out if this is common among many mirror lenses. – lijat Feb 25 at 4:02

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