Why do mirror lenses often show lower contrast?
Asked 4/5/2012
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I understand why mirror (catadioptric) lenses produce donut-shaped bokeh and why they usually have a fixed aperture, but why do they often look lower in contrast than refracting telephoto lenses? Is the reduced contrast caused mainly by the central obstruction, or by something else in the optical design?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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The loss of contrast is due to the central obstruction; that is, the "hole of the donut" that blocks light from getting through the center of the lens.
In a diffraction-limited telescope or lens, the point spread function—basically an image of a point source, like a star—is the Fourier transform of the aperture. For a circular aperture, like most lenses, this is a Bessel J1 function, also known as a Jinc, or an Airy disk:
When you obstruct the center of the aperture, turning it into a donut, the Fourier transform changes in a way that transfers power out of the central bright spot, into the first ring. The effect is to lower the contrast of the image.
This web page has examples of the point spread function for all sorts of obstructed apertures, so you can see the effect clearly.
Originally by user2138. Source · Licensed CC BY-SA 4.0
user2138
14y ago
0
Generated from our catalog & community — verify before relying on it.
Mirror lenses can lose contrast for two related reasons.
First, the central obstruction changes the aperture from a full circle into a ring. That alters the point spread function, moving some light out of the central peak and into surrounding rings. The result is reduced local contrast, which can make fine detail look less crisp.
Second, the more obvious “foggy” or washed-out look often associated with mirror lenses is commonly attributed to increased internal scatter from the reflective optical surfaces. A normal refracting lens also scatters some light, but typically less. So while the obstruction affects sharpness and microcontrast, the overall low-contrast rendering many people notice is not explained by the obstruction alone.
In short: the central obstruction lowers local contrast, but the characteristic overall low contrast of many mirror lenses is generally linked more to light scatter in the mirror-based design.
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