What is a "mirror" lens?

[edawn]

I've seen some lenses referred to as mirror lenses, and I wasn't sure what exactly that meant, so I thought I'd ask the wise folks at photocamel...

Elaine

[scoundrel1728]

A mirror lens is just what its name says: the heavy light bending is done with mirrors rather than lenses, though they may have lenses at the tail end to allow for some optical tweaks. Typically these lenses have two mirrors in them, the main mirror being a concave parabolic reflector and a secondary mirror that may be flat or convex to bounce the light from the mirror back toward the camera.

Mirror lenses are long telephotos because the image quality of mirror lenses degrades quickly as you deviate from the optical axis. Your typical mirror lens is a 500 mm f/8, though Sigma makes a 600 mm f/8 and there are (more expensive) 1000 mm f/11 models out there. They are typically not very bright either, again because of their poor off-axis performance. Because of the central return mirror, these lenses don't normally have diaphragms as most lenses do. In other words, their aperture is fixed. You can't stop them down in the conventional way, but I have heard of one person who cut out some cardboard aperture stops, the apertures being off the optical axis to avoid having the return mirror block the optical path. Also because of the central return mirror, out-of-focus specular highlights are ring-shaped instead of being circular or polygonal as they would be in a more conventional lens. This makes for a distinctive (AKA strange) bokeh.

However, these lenses have their advantages too. In a "pure" mirror lens, chromatic aberration will be zero because the reflective path, unlike a refractive one, is independent of wavelength. The light path is also folded for a good part of the lens' length so a mirror lens will be a lot shorter than its focal length. Because you don't need a lot of glass to make these mirrors, the lenses are a lot lighter than their more conventional counterparts and the main element, the objective mirror, is near the lens mount where it can be well-supported and won't shift the camera's center of mass too far forward. And they are comparatively inexpensive.

[edawn]

Thank you very much! Yes, I was noticing that they only seemed to be long telephotos and that they weren't very expensive. Thanks for the lesson!

Elaine

[Jose]

I have a mirror lens as the one described in the link below that I seldom use now but back in the 80's it was fun to play with. When used, it is as a telescope with a 20X Vivitar attachment to look at distant subjects such as comets when they appear.

http://www.luminous-landscape.com/re...olid_cat.shtml

[bircher]

Anyone else have any experience with mirror lenses? I saw one at B&H for $100 and I thought it would be fun to play around with.

Sam

[Lin Evans]

They can often be a very inexpensive alternative to a "very" expensive optic. Because there are no chromatic aberrations - unless induced by the eyepece - mirror type telescopes often make great lenses for seriously long focal length photography commonly referred to as "digiscoping."*

Digiscoping is best done with a relatively slow and small diameter objective. Some of the best digiscoping cameras every built were the early CP series Nikon such as the CP800, 950, 990, 995, 4500, etc. Using them in an afocal manner (shooting through the eyepiece of the telescope) it's often possible to get very good to excellent images at ridiculous focal lengths.

When using a dSLR or 35mm film SLR the eyepiece of the telescope is left out of the equation and specialized camera ports are accessed via a "T" connector and "T" adapter. In such cases the focal length is limited to the actual native focal length of the barrel of the scope. If you use a crop factor camera the "effective" field of view will be the crop factor time the focal length. For example a 1.6x crop factor Canon dSLR used with a 1250mm Meade ETX-90 renders a 35mm equivalency of 2000 mm.

Because afocal (through the eyepiece) use with a fixed lens digicam adds the power of the eyepiece it's possible to shoot at focal lengths of up to 6000 mm and sometimes more. Below are a three samples. The first - a Starling taken afocal with my Nikon CP990 and Meade ETX-90 through a William Optic 24mm eyepiece yields 5989 mm. The second (robin) taken with the same camera is zoomed back to 2300 mm. The third (moon) and fourth (tiny butterfly on dandelion) were taken with my Canon D30 and Meade ETX-90 at an effective focal length of 2000mm. The Mead ETX-90 has the added ability of being able to focus as close as seven feet - something simply not possible with high powered and expensive camera optics. The tiny butterfly was taken from a distance of about 8 feet again at an effective 2000mm.

As you can see the detail is excellent. The scope has a fixed focal length of 1250mm and a fixed aperture of about F14 which is sufficient for decent DOF and crisp images. These combinations can result in the ability to get frames you simply can't get with conventional equipment so have their place in a photographer's tool box.

Lin

[bircher]

Lin,

Thank you for that information. It was very helpful in making my decision. I am assuming the shots that you have included here were with a tripod. Have any luck hand holding your mirror lens?

Sam

Originally Posted by Lin Evans

They can often be a very inexpensive alternative to a "very" expensive optic. Because there are no chromatic aberrations - unless induced by the eyepece - mirror type telescopes often make great lenses for seriously long focal length photography commonly referred to as "digiscoping."*

Digiscoping is best done with a relatively slow and small diameter objective. Some of the best digiscoping cameras every built were the early CP series Nikon such as the CP800, 950, 990, 995, 4500, etc. Using them in an afocal manner (shooting through the eyepiece of the telescope) it's often possible to get very good to excellent images at ridiculous focal lengths.

When using a dSLR or 35mm film SLR the eyepiece of the telescope is left out of the equation and specialized camera ports are accessed via a "T" connector and "T" adapter. In such cases the focal length is limited to the actual native focal length of the barrel of the scope. If you use a crop factor camera the "effective" field of view will be the crop factor time the focal length. For example a 1.6x crop factor Canon dSLR used with a 1250mm Meade ETX-90 renders a 35mm equivalency of 2000 mm.

Because afocal (through the eyepiece) use with a fixed lens digicam adds the power of the eyepiece it's possible to shoot at focal lengths of up to 6000 mm and sometimes more. Below are a three samples. The first - a Starling taken afocal with my Nikon CP990 and Meade ETX-90 through a William Optic 24mm eyepiece yields 5989 mm. The second (robin) taken with the same camera is zoomed back to 2300 mm. The third (moon) and fourth (tiny butterfly on dandelion) were taken with my Canon D30 and Meade ETX-90 at an effective focal length of 2000mm. The Mead ETX-90 has the added ability of being able to focus as close as seven feet - something simply not possible with high powered and expensive camera optics. The tiny butterfly was taken from a distance of about 8 feet again at an effective 2000mm.

As you can see the detail is excellent. The scope has a fixed focal length of 1250mm and a fixed aperture of about F14 which is sufficient for decent DOF and crisp images. These combinations can result in the ability to get frames you simply can't get with conventional equipment so have their place in a photographer's tool box.

Lin