Marc,
Here is a piece on advice on long focal lenses. I have quite a lot of them, including some crappy lenses and too few very good ones, because my budget is rather limited.
First, about mirror lenses. We all know that the F-stop value of a lens is the theoretical aperture calculated by dividing the focal length by the diameter of the lens. For most lenses, this is a good approximation of the actual measured aperture (T stop). But not for mirror lenses, which have a secondary mirror located right in the middle of the front element. This secondary mirror and the transmission losses due to the less than perfect reflections on the mirrors make the T-stop about 1 stop slower than the F-stop. In other words, the actual aperture of a 500mm F/8 mirror lens is T/11. Even a "fast" 500/6.3 is in fact a T/9.1 lens.
One of the main arguments in favor of mirror lenses is that they are small and lightweight. Maybe, but they also have a very limited aperture compared to refractive lenses. Have you ever seen a Russian 500/5.6 lens? I wouldn't call it small or lightweight, and it's only a T/8.0 lens! That said, mirror lenses can have quite acceptable image quality. Due to their particular construction, mirror lenses have no or very little chromatic aberrations. I suggest you look for a relatively short focal length, with the fastest aperture you can find, as this kind of lens will be much more usable than either a long focal length with a very limited aperture, or an enormous lens with an acceptable aperture. I have a Tamron 350/5.6 mirror lens (actual aperture: T/8.0), which is a very sharp lens. The famous "donut" bokeh of mirror lenses can produce some spectacular images if used with taste. Here are some pictures of mine, taken with the Tamron 350/5.6:
Second, about the el-cheapo supertele lenses. They come in many flavors (Phoenix, Quantaray, etc.). The most common are either the 600-1000 zooms or the 400/6.3 and 500/8 prime lenses. They are cheap, because they have simple optical construction (usually an achromatic doublet, that is 2 elements in one group for the prime lenses) and they do not feature any fancy low dispersion glass. They are not much heavier than mirror lenses but image quality is not very good, mostly due to chromatic aberrations. If you are interested in these, I suggest you look for the better ones, like the
Novoflex Noflexar (most common are 400/5.6 and 600/8) or the
Leitz Telyt (most common are 400/6.8 and 560/6.8). The Noflexar and Telyt lenses are both quite long as they are long focal lenses and not telephotos, but the lens body can be dismantled in 2 or 3 pieces for transport. Check the
website of master wildlife photographer Douglas Herr to get an idea of what it's possible to achieve with a Leitz Telyt (and those are not as expensive as you might think... I just got a 400mm with T-mount adapter for $350 on eBay).
The last reasonable option option IMHO is to buy a modern telephoto lens with low dispersion glass. You will get the highest image quality and fastest aperture. Of course, the price tag will make you faint, especially for lenses over 300mm. The only Pentax lens I could (barely) afford in this category was the F* 300/4.5 (I paid about $1000 for it), which is certainly my best telephoto by far (and also my most expensive one): superb image quality, lovely colors, ultra low chromatic aberrations... I also have the Tamron Adaptall 2 300/2.8 telephoto (bought it for about $450 from Keh a few years ago). It's a nice lens, fast and well built, but still a notch below the Pentax in image quality. As it is a fast lens, I use it mainly with the Pentax 1.7X AF adapter, which turns it into a 510/4.5 autofocus lens. But the main reason I don't use it as much as the Pentax is that it is too bulky and heavy to handheld or to carry into my hand luggage for plane travels.
And what about the older telephoto lenses? I have a pair of old fashioned behemoths: the Takumar 500/4.5 and the Tamron 200-500/5.6. I don't use them much. Both have acceptable resolution, but they do not feature low dispersion glass, which means they exhibit a lot of chromatic aberrations wide open. So you have to close down the aperture at least 2 stops to get good image quality, which means shooting at F/11. Then why not use the Pentax 300/4.5 + 1.7x teleconverter or the 400/560mm Leitz Telyt instead, which are both much lighter and easy to carry than the aforementioned prehistoric beasts?
That's my 2 cents on the matter.
Cheers!
Abbazz
Last edited by Abbazz; 05-14-2009 at 10:19 PM.
Reason: Fixing links