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
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.
chema 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 dounot-like effect on the out-of-focus high-lights. The following picture illustrates this problem.
working distance ~10m
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
Artical : http://www.photozone.de/mirror-lenses