Mirror Lenses
Technology - Technology
If you're on a budget and intend to shop around for a really long tele lens you may have noticed a quite strange rep of the species - mirror lenses. For photographic usage there're usually 500mm f/8 mirrors but there're also faster versions like 500mm f/5.6 and with extreme focal lengths like 1000mm f/11. Naturally mirror lenses are also widely used for astronomic purposes.
Mirror lenses are comparatively small - e.g. a 500mm f/8 mirror is about as "big" and light-weight as a mid-range standard zoom. The focal length is extremely interesting for wildlife photography and the price is usually hot - very hot. At around 200-400 Euro/$ this is certainly the very cheapest option in this range. So is it crap or really something worth to be considered ?

In order to impress you here's a shot with a Tokina 500mm f/8 mirror lens:

~2m working distance

Looks pretty good, doesn't it ? But it tells only part of the story - the shot above was taken under optimal conditions. More on this later.

Let's have a look at the principal design of a mirror lens first. As the name implies these lenses are mainly based on mirrors rather than glass elements. Incoming light gets reflected by a main mirror (located at the back of the lens)  towards a small secondary mirror (at the front) which then reflects the light back towards the film via a correction (glass) element. The lack of glass elements (apart from the correction element) is a significant design advantage compared to classic (refractive) lenses - chromatic aberrations (visible as color shadows) are virtually absent. Looking at the illustration below you may notice that the light path gets folded. This is the reason why these lenses offer such a long focal length while keeping a very small length. Dependent on the configuration the main mirror can be pretty large so while quite short most mirrors tend to be rather fat as well. 500/8 mirrors remain pretty compact but e.g. 500/5.6 or 1000/11 mirrors are about as large in diameter as the width of an SLR.

schema of a Schmidt-Cassegrain-type mirror lens

 Unfortunately the design results also in some significant disadvantages like
  • no aperture so it's not possible to control the depth-of-field. 
  • the mirror reflections eat contrast
  • the overall sharpness is usually quite mediocre
  • the secondary mirror produces  an odd effect donut-like effect on the out-of-focus high-lights. The following picture illustrates this problem.

working distance ~10m

Typically Mirror lenses are quite slow so you may live with the limitation of a fixed aperture. The contrast performance of a good mirror is roughly comparable to a typical xx-300 consumer zoom at the long end so that's not TOO bad (some mirrors are quite unacceptable in this respect though). However, the unsteady background blur is disturbing indeed and kills the lens for professional purposes. At first it may be interesting but sooner or later you recognize that it draws the viewers attention off the main subject.

As mentioned above there're some situations where you can avoid or at least reduce this donut effect. You've to have a quite even background, an isolated main object and a close focus distance. The picture at the top of this document is an example for this. It was taken at a ~2m distance and avoids any highlighted objects both in the fore- and background thus resulting in a perfect blur.  The picture below looks still pretty good with a working distance of maybe 5m and a little steeper view towards the ground - you may already notice some minor hints of donuts here.

~5m working distance
Conclusion: Yes, mirror lenses produce pictures and under carefully chosen conditions they may even produce pretty good ones. However, these situations are rare. For example the shots above were taken in a local bird zoo where you can get very close to your subject - in real life you'll most likely end up with situations like in the eagle picture (the 2nd picture) and therefore with unsteady background blur. Most mirror lenses suffer from a rather mediocre optical performance. This may all be fine for personal purposes and for documentation but most likely you'll not be able to sell such pictures. Therefore a classic (refractive) lens like a 400/5.6 fix-focal or even a xx-500mm zoom is usually a better solution for a more serious approach. Nonetheless mirrors are quite cheap and  fun to play with.

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