Why do mirror (catadioptric) lenses usually have a fixed aperture?
Asked 10/13/2016
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Catadioptric or mirror telephoto lenses use mirrors, often like small telescopes, to achieve long focal lengths in a compact body. Many classic photographic mirror lenses are fixed-focus and fixed-aperture. I understand why the focal length is fixed, but why is the aperture usually fixed as well? What is it about the optical design that makes a variable iris difficult or impractical in these lenses?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
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The mirror lens has an advantage in that it is completely free from chromatic aberrations. All other lens systems are degraded by longitudinal and transverse chromatic aberrations. These are brought about as the image forming rays pass through a transparent lens. The mirror lens avoids this because the mirror’s silvering is on the surface of the lens; thus the image forming rays never enter. Thus the mirror lens is able to deliver a well-defined image with high magnification.
Another benefit of the mirror lens is its short barrel. This is achieved by causing the image forming rays to double back on themselves. This is possible as the main or objective mirror lens is placed at the rear of the tube. The objective mirror’s shape causes the rays to converge. These travel to the front of the tube where they encounter a smaller secondary first surface mirror. This secondary mirror directs the rays to the back of the tube. These image forming exit the tube via a hole drilled in the center of the primary mirror. The rays exit and an image is projected on film or digital sensor.
This is a catadioptic system that features a shortened barrel free of chromatic aberrations. The system is however not free from the remaining six aberrations that plague all optical systems. The catadioptic system features a thin transparent lens at the tubes entrance. This lens is used to make some corrections that mitigate some of the aberrations. Thus this corrector lens allows for a simpler shape (figure) of the objective mirror.
The key to the folded (shortened) optical path is the up-front second mirror. The problem is, this secondary mirror blocks a significant amount of light that would have entered if the systems were a conventional transparent lens design. The shadow of this obstructing secondary mirror will image if the lens is stopped down to the smaller f/numbers (tiny diameter). Additionally the obstructing secondary makes it almost impossible to install a mechanical iris diaphragm. We are forced to control exposure via shutter speed or ISO setting or both.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
9y ago
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Generated from our catalog & community — verify before relying on it.
Mirror lenses usually have a fixed aperture because of how the optical path is arranged. In a catadioptric design, light reflects between mirrors and passes through a central obstruction caused by the secondary mirror. That already creates an annular, not normal circular, entrance pupil. A conventional iris diaphragm is hard to place in a useful location without blocking the folded light path or upsetting the design.
Stopping the lens down also tends to remove one of the main advantages of a mirror lens: compact size at a long focal length. Instead of improving performance in the same way as a refractive lens, an iris in a mirror lens can introduce additional vignetting/diffraction and further reduce light transmission in an already relatively slow lens.
Because the optical geometry is tightly constrained, most photographic mirror lenses were designed around a single effective aperture that balanced size, brightness, and image quality. Some designs have used alternative methods such as rear-mounted filters or special controls, but the classic consumer mirror telephotos are typically fixed-aperture for practical optical and mechanical reasons.
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