Mirror Lenses.

CT

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Also known as Cat or Catadioptric lenses.

What is a mirror lens? Well it's a lens where the image is bounced back and forth off internal mirrors to achieve a longer focal length than the lens body would normally allow. Compared to a normal lens, mirror lenses are both short and usually extremely light. They're commonly available in 500mm and 600mm focal lengths (there have been longer versions.... up to 2000mm) and there's usually a good selection available on ebay for a fraction of the cost of a normal tele lens.

"Fantastic!" I hear you say, "What's the catch?". Well you guessed...there are a few actually.

1. There is no diaphragm (aperture) mechanism in these lenses, they come usually with a fixed aperture of f8, although there are better ones available with an aperture of f5.6.

2. Out of focus highlights take on a pronounced 'Doughnut' appearance, which isn't always necessarily unattractive, but it is a dead give-away that a mirror lens has been used. Some conventional tele lenses can produce the effect under certain circumstances, but nowhere near as pronounced as with a mirror lens. Creamy out of focus backgrounds are not the norm with these lenses unless you can take your shots against a fairly plain background.

3. Edge definition doesn't hold up well with these lenses compared to a normal tele lens, but the better ones can perform well at the centre when pitted against quality tele lenses, although the latter will emerge triumphant, it's often a close call.

Three interesting articles to read which will give you a good idea of what's involved....

http://www.photozone.de/3Technology/lenstec5.htm

http://www.photo.net/learn/optics/mirrors/

http://www.luminous-landscape.com/reviews/lenses/solid_cat.shtml


Few people would really recommend a mirror lens, over a normal tele lens but if your budget is limited and/or you occasionally need a long lens, but not often enough to justify the expense of a normal long lens, then for a modest outlay, a mirror lens might be the answer for you. On the plus side they're light and hand-holdable. F8 is a restriction, but with a budget long lens of the same length, which would cost a fortune in comparison, you'd be unlikely to have a max aperture of more than 5.6, so there's only one stop difference in it.
 
Thanks for that, great info, I had wondered what they were for.
 
Often thought 'what's the point of these things' ... have seen some great arty type pics produced from them ... but that's about all really ...:p
 
I have a mirror lens for my old AE1 Program, Tamron one, is F8 and i did actually used to get some not bad pics with it, even with a 2x converter on it ! It sits here in my old camera bag with the AE1 and a couple other lenses ! lol Maybe i should sell it ! lol
 
A happen to own a Russian 1000mm f11 mirror lens, it makes a reasonable telescope when fitted with an eyepiece adaptor but as a camera lens it's soft soft soft!
 
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