Mirror Lenses

[klindup]

Hi Bob
Just read your article on mirror lenses and a couple of questions came to mind.  Instead of using a TC, could you use a Barlow or a Televue Powermate?  Would there be any advantage over a TC?
Second question relates to the lens.  Would you be better using a small SC telescope On an altar mount or even a Maksutov such as a Meade?
I know we are talking unrealistic silly prices in this context but I have seen great results using a Questar. They even used to advertise what you could do using one as a wildlife lens.  Not a realistic option I know on cost basis but it demonstrates what a catadioptric device is capable of.
Ken

[Bob Atkins]

To the best of my knowledge, they are both essentially the same thing. They are a negative lens system - which effectively increases the focal length of the primary lens. I'd guess the main difference is that they are designed for different applications. Barlow lenses are typically used visually and don't need a wide field, while TCs are designed for full frame 35mm coverage. There are probably barlow designs that will cover a wide field and there are certainly higher power barlow lenses (up to maybe 4x or 5x), but the principle of the barlow lens is the same as that of a TC.

The answer about SC telescopes vs mirror lenses is again lost in the detailes. They are the same thing optically. Most mirror lenses are SC, though there are, I think, some Rusian Maksutov designs. Lenses will typically have some sort of field flattening optics that some (most) telescopes lack - but you can often buy external field flattening optics for photographic work. For a scope you want maximum visual brightness (minimum number of elements) while for a lens you want maximum field correction over a full 35mm field.

I've used a Teleview 500mm f5 (100mm) refractor as a lens, and it's a great lens. Long, heavy and inconvenient as a lens, but excellent optical quality. I'm sure you could use a Questar for excellent images too, though as you say, it's not exactly a cheap way to go!

So if you have a telescope, it's not a bad idea to try using it as a lens. However if what you want is a lens, then I think you're typically better off buying a lens - and a refractive lens if possible. The only use I see for mirror lenses is if you need something really small and light (e.g. Tamron 500/8 mirror). The only time a mirror telescope might be the instrument of choice is if you want something with a really long focal length. Maybe something like a an f10 8" SC which would give you a 2000mm f10 lens (through I don't know if will cover a full 35mm or APS-C frame).

[klindup]

I take your point about telescopes being so heavy and bulky and that lenses designed to be used on cameras are the best option.  I also have a Televue NP 101 bought for observing but which makes a great 540mm f5.4 lens if the walk from the car is but a few yards.  The weight of the telescope and the Gibralter mount needs two trips to set up and a third to fetch the camera.  However if I can find a suitable site the images I get of birds are great and worth the effort.  But not the first choice when looking for a 500mm lens.

[Bob Atkins]

Mirror lenses can be pretty good - as well as small, light and inexpensive. They can also be not so good.

See my review of the Pro-optic 500/6.3 lens  http://www.bobatkins.com/photography/reviews/pro_optic_500_f6-3.html

It's cheap ($160 when tested) and nominally reasonably fast ("f6.3"), but unfortunately it's not all that sharp and the T-stop is closer top f10 than f6.3.

The Tamron 500/8 (discontinued) is much better, plus it's actually smaller and around the same price (if you can find a used one). If you must have a long lens and can live with the limitations of a mirror lens (fixed, slow, aperture plus manual focus and "do-nut" out of focus highlights), it's not a bad choice.

[Frank Kolwicz]

It seems to me that most people think about mirror lenses when they either are looking for a small, light, cheap alternative to expensive, heavy, long focal length camera lenses or are looking for extreme magnification, 1200mm plus. I’m not sure what use they have in mind for the images, as the out-of-focus highlights (the bright doughnuts) are generally considered rather ugly, a kind of ultimate anti-bokeh, that makes the images mostly unsuitable for exhibition or publication. (This is my opinion, after all, not the 11th commandment.)

Once you take into consideration that mirror lenses are manual focus, you find that your choice of comparable photographic refractors with the same limitation and at about the same price extends your choice to many old and therefore cheap models from several manufacturers. The reasons then for going with a mirror lens become much smaller – essentially light weight and compact size. The fixed and relatively small aperture is another hindrance that can be avoided with a photographic lens.

There are so many choices for proper photographic lenses in the under-600mm range that would be equal or superior in use to a mirror lens, that I can only imagine one use for a mirror lens: for extreme back-packing or trekking to record observations of a scientific or personal nature where image background and out of focus quality is of no concern.

As to long focal length uses, the description of the cost of and difficulties handling the big mirror lenses previously described should be enough to put off any consideration of them as replacements for camera lenses of about 1200 mm or more. Beyond 1200mm there are also difficulties getting sharp images due to camera movement and atmospheric aberrations under all but ideal conditions. The highlight doughnuts everywhere in the image (since almost all of the image will be out of focus) is a killer – I know, I’ve been using a Canon 600 with 2x converter with a 7dii for birds (effective focal length 1920mm) and atmospheric aberration at distances where the long focal length was needed has wasted all images on some days and most images, most days. Only when the atmosphere was extraordinarily calm could I get reliably sharp images at that focal length. The longer the focal length, the worse the problem! And, don’t forget, no image stabilization, either.

All in all, there are good reasons why mirror lenses are of little interest for terrestrial photography.