Are mirror lenses often sharper at the edges than in the center?
Asked 2/24/2020
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I saw a review of the Samyang 300mm f/6.3 mirror lens claiming it looked sharper in the corners than in the middle. Is that typical behavior for mirror/catadioptric lenses, and if so, why would the center appear less sharp?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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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.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
6y ago
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Generated from our catalog & community — verify before relying on it.
It can happen, but it is not a universal rule for all mirror lenses.
Mirror (catadioptric) lenses have a central obstruction from the secondary mirror. That obstruction reduces contrast, and lower contrast can make the image look less sharp even if detail is still present. This is also related to the characteristic donut-shaped out-of-focus highlights seen with mirror lenses.
Another factor is field curvature and focusing. A lens can sometimes be focused so the outer areas of a flat test target look best, which can leave the center looking worse by comparison. If the optical design is compromised for cost or simplicity—such as relying on spherical rather than more ideal mirror shapes—that can also affect how sharpness is distributed across the frame.
So: edge-or-corner sharpness exceeding center sharpness is possible with mirror lenses, especially in certain designs or test setups, but it should be understood as a result of their optical design and focus behavior rather than as a guaranteed trait of every mirror lens.
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