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I understand that mirror lenses already have high maximum apertures and that often, the last thing one wants to do is let in less light, but is there a physical reason that one could not build a working mirror lens that did use a variable aperture? Is it impossible to put the aperture in the right place? Is there something about the optical pathway that would ruin the functionality of an aperture. The advantage would be that one could get greater depth of field.

  • I have a vague recollection of someone linking to a variable aperture mirror lens on here not so long ago. I'm sure if I'm not mistaken, this lens will be mentioned again presently by someone familiar with it... – osullic Mar 16 '17 at 22:16
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    Indeed, here it is – osullic Mar 16 '17 at 22:41
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There is no real theoretical reason a mirror lens can not have a variable aperture. In fact, many Newtonian reflector telescopes do have a primitive adjustable aperture for use when viewing very bright objects such as the moon.

This homemade one has three holes of different sizes. In normal usage two of the holes would be covered and light would only pass through the selected diameter hole to control the amount of light collected from bright astronomical objects. Such apertures only affect brightness, though. Depth of Field isn't really affected.

homemade variable aperture

The lack of any catadioptric mirror lenses that are made with adjustable apertures are practical reasons.

Most mirror lenses made for photographic usage are catadioptric in their design. That means they combine some of the properties of a reflecting mirror with some of the properties of a refractor lens. Designers do this to create a lens with very long focal length in a form factor much more compact than a conventional refractive lens of the same optical power.

In most cases the front lens element is either a flat dust protector or a "corrector" plate that is either an aspherical refracting lens or a more simple meniscus refracting lens. For lenses with a front corrector plate, the light enters the front of the lens, is refracted by the front corrector plate, is reflected by the primary mirror at the back of the lens onto the secondary mirror located in the center near the front of the lens just behind the corrector plate and then reflected by the secondary mirror through a hole in the middle of the primary mirror at the rear of the lens. Lenses with only a flat front protector plate usually have correcting lens element(s) just behind the primary mirror that refracts the light passing through the hole after it is reflected by the secondary mirror. Lenses with a corrector plate may or may not have additional correcting lens elements behind the primary mirror.

Aspherical Corrector Plate design

Since the secondary mirror (which is sometimes only a silvered surface on the center of the rear side of the meniscus corrector plate in the classic Maksutov–Cassegrain design) requires an obstruction in the center of the lens, a conventional internal diaphragm iris is not possible with such a lens. An external aperture attached to the front of the lens, similar to the one for our Newtonian reflector above, could be used with a flat front plate design. But it would be unwieldy and increase the size of the lens. This defeats the whole purpose of a mirror type lens which is high focal power in a compact design.

Minolta AF 500mm mirror lens

Such an aperture would not affect DoF in any meaningful way, though. Just as with the Newtonian reflector above, the aperture would only affect brightness. DoF is defined by the F-number which is based on the mirror diameter in a catadioptric lens design. The brightness is defined by the T-stop, which is reduced by such an aperture, just as it is reduced by the permanent center obstruction.

There has been at least one mirror lens that did incorporate a manual aperture iris at the very front of the lens. It was available in many popular SLR mounts around in the 1970s and 1980s. In this position the aperture setting only affected the T-stop number of the lens, but not the depth of field. The Ohnar 300/5.6 Mirror (also sold under other names such as Hanimex, Makinon, Panagor, etc.):

Ohnar 300/5.6 Mirror

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    Well put. There's also the minor issue of any such aperture killing either the low- or high-spatial frequency info,but compared w/ usual central obscurations in cat lenses, that's a minor issue. – Carl Witthoft Mar 17 '17 at 12:40
  • Obstructing the clear aperture necessarily has an impact on the depth of field, as you have stopped the lens down. Using the diameter of the primary mirror in a newtonian telescope assumes the aperture stop is at the primary mirror itself. By adding an aperture elsewhere, you have moved the stop and closed the aperture. – Brandon Dube Mar 17 '17 at 19:12
  • @BrandonDube And thus there are qualifications in the answer. "Such an aperture would not affect DoF in any meaningful way, though." What is considered meaningful on the lab bench may not be significant in the real world practice of creative photography, which is what the scope of this group is supposed to be about. The loss of high spatial frequency with actual catadioptric camera lenses available in the marketplace makes minor differences in DoF fairly meaningless in practice. As anyone who has ever actually used and reviewed the Ohnar or its clones has noted, stopping down has very... – Michael C Mar 18 '17 at 1:28
  • ... little to no effect on perceived DoF. – Michael C Mar 18 '17 at 1:28
  • Why is your tone so defensive? – Brandon Dube Mar 18 '17 at 1:39
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Is there something about the optical pathway that would ruin the functionality of an aperture.

The aperture is necessarily a hole in the middle of the mirror. The larger the aperture, the less room there is for mirror surface without making the lens larger. If you don't mind carrying around a lens that's longer and wider, then perhaps you could afford to put a larger hole in the middle, but mirror lenses are already fat enough to be unwieldy.

The advantage would be that one could get greater depth of field.

I think you've got things backward: smaller apertures (higher f-numbers) give more depth of field. Larger apertures (smaller f-numbers) allow more light but give less depth of field.

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    A lens with no variable aperture is already at the widest possible aperture for its design. Any change in aperture would be in the direction of restricting light entering the lens and would be to reduce the size of the aperture (increase the f-number) which would give greater DoF at the expense of image brightness. To increase the size of the aperture one would need to increase the size of the entrance pupil, which would almost certainly require larger lens/mirror elements. – Michael C Mar 16 '17 at 22:23
  • Why is the aperture necessarily a hole in the middle of the mirror? Do curved mirrors (either spherical or parabolic) act the same way as refractive lenses when light is blocked in an asymmetrical pattern? – Michael C Mar 16 '17 at 22:34
  • I've got a fixed f/8 mirror lens as my longest telephoto. My biggest complaint about it is the shallow depth of field -- the hyperfocal distance is 1500 meters, and at extreme close focus, the depth of field is only 2 mm thick. – Mark Mar 16 '17 at 23:23
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    Pretty much any lens at MFD has a very narrow DoF. – Michael C Mar 17 '17 at 0:18
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    @MichaelClark As you probably know, there are off-axis parabolic primaries which get rid of the central obscuration due to the secondary, but since they're off-axis, there's significant problems with coma over FoV. – Carl Witthoft Mar 17 '17 at 12:41
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In most catadioptric 'lenses,' the primary mirror is the aperture stop. It is quite a large surface, and because of the tighter tolerances around mirrors mechanically crowded. Large diaphragms are expensive and it would be difficult to implement a mechanism to control a diaphragm inserted on top of the primary mirror.

A makutsov design can be changed where the secondary mirror is the aperture stop, but that has worse coma, astigmatism, and distortion than the stop being at the primary mirror. A better performance at the full aperture is usually considered better than a worse design which you can close the aperture of.

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