My most viewed photo on flickr by a significant margin (8300 views vs 2900) is, somewhat annoyingly, a picture of my 20D with a Sigma 600mm mirror lens attached. Since almost all the referrals come from search hits from one place or another there’s clearly a lot of interest in mirror lenses. Most of this I assume comes from people who have seen that mirror lenses seemingly offer huge focal lengths for not much cash compared to traditional alternatives for the same focal length. Everyone loves a bargain, right? So why pay thousands when you can get the same focal length for just a ton or so? Well here’s my answer – you get the same focal length but you get nowhere near the same quality.
But let’s take a step back for moment, and clarify what we’re talking about. What is a “mirror lens”, anyway? It’s not entirely a lens, and it’s not entirely a mirror – technically it’s a catadioptric optical system that combines both refracting lenses and reflecting mirrors. Basically light enters the front of the lens usually via a correcting plate, and strikes a concave mirror which focuses and reflects it back up the tube, where it strikes a smaller secondary mirror in the centre of the correcting plate. The secondary focuses and reflects the light again, back through a hole in the centre of the first mirror, where a lens group focuses the image on the film/sensor plane. This has both advantages and disadvantages compared to a long telephoto of normal lens-only construction.
Firstly there’s the construction – as an example Canon’s current 600mm f/4 has 17 elements in 13 groups, the previous, non-stabilised version had 9 elements in 8 groups, but they both weigh over five kilograms and are nearly half a metre long. By contrast my Sigma mirror has only 7 elements in 4 groups, weighs 860g and is only 122mm long. So that’s two obvious advantages right there, size and weight, but also cost – it’s clearly less expensive to make fewer elements. Also it’s easier to make a high quality optical mirror, where only the surface needs to be accurately prepared, than a high quality lens which needs two surfaces and a flawless interior. Both the size and weight saving provide a third bonus – it’s much easier to hand-hold a mirror lens. The last real advantage is that unlike refraction, reflection is not wavelength dependent so it affects all colours of light equally, and so chromatic aberration is much reduced in a mirror lens.
So, on with the negatives. Firstly, the position of the secondary mirror at the centre of the lens causes a problem – it obstructs some of the entrance pupil which has a number of effects. First it cuts down the total light gathering capability, although maybe not by as much as you’d think – maybe half a stop. More critically it prevents the use of an iris diaphragm as a variable stop in the optical path, and so mirror lenses are all fixed aperture. The only was to reduce the light going through is to use an ND filter at some point. Lastly because of the annular entrance pupil, out of focus highlight take on a characteristic doughnut shape. While sometimes this can work – if for example you want to illustrate an article on Mysterons – often it’s unwelcome and ugly and because of the long focal length and ultra-thin depth of field, hard to avoid.
A related problem is that inevitably, the fixed aperture is not very wide; my 600mm is f/8 which makes for a pretty dark viewfinder. Since almost all mirror lenses are manual focus (only Minolta/Sony made/make an AF mirror) this is not helpful. It also means that on even bright sunny days, you need to bump up the ISO to get a reasonable shutter speed, even if you give up on the idea of hand-holding and use a tripod.
Another problem stems from the folded optical path – because the light travels up and down the lens tube three times three times rather than just once there is more likelihood that stray light can get inside and bounce around, reducing contrast. To some extent you can counter this in post processing, but it does lead to muddier colours.
But all that is rather theoretical, and doesn’t cover the main issue which is the final image quality. I can only speak for my own example in this regard, of course, but what I did some time ago now, was to shoot a number of targets with both the 600mm mirror, and with my 70-300mm zoom. I then cropped the 300mm shots and up-sampled them to the same size as the mirror frames for a series of direct comparisons. What this showed was that really you’re not gaining much, if anything, over simply cropping and resizing shots from a good 300mm zoom lens, and the advantages of the zoom in this case include image stabilization and autofocus. I would imagine that a comparison with a 300mm prime would go overwhelmingly in favour of the lens rather than the mirror, however, obviously we then return to the issuse of price, where the mirror is usually significantly cheaper. In my case the Sigma 600mm cost less than a quarter of the price I paid for the 70-300mm zoom.
Trying to wrap this post up, I’d say if you have a good 200 or 300mm lens already, then it’s probably not really worth deliberately seeking out a mirror lens, especially if it’s a make you’ve never heard of. On the other hand if you come across one loitering in a car boot sale, or charity shop for a bargain, they can hold their own and be a bit of fun.