I'm wondering why you would be interested in "short" focal length mirror lenses.
These lenses fulfill a very special purpose and you would normally not want to use one for general purpose photography.
A "mirror" lens (technically these are "catadioptric" lenses which means it uses a combination of lenses and mirrors. The front piece of clear glass is a "corrector plate" and is technically a lens even though the corretion is minor.
These lenses have no ability to communicate with the camera -- they have a fixed focal ratio which cannot be changed. They are manual focus only.
They commonly come in 400mm, 500mm, or 800mm focal lengths.
When a lens uses glass elements, the element works by bending light. Whenever you bend light by passing it THROUGH a lens element (as opposed to using a mirror), the light refracts based on it's wavelength. Red light does not bend as much as blue light for example. This means a simple single lens element cannot focus all wavelengths of light to the same focal plane. To correct for this, additional lens elements are included -- often MANY lens elements.
When lenses get longer, the amount of light making it to through decreases -- and to compensate, the lens needs to have a much larger physical diameter. When using lens elements, these elements also need to be physically larger. As the lens elements get larger, the prism effect which causes dispersion (different colored wavelengths bend different amounts) becomes even more extreme. This is why VERY long focal length lenses become VERY expensive.
"Mirror" lenses side-step the whole problem by bouncing the light off mirrors... there is no dispersion when bouncing a light off a mirror (you are not technically passing light "through" a lens element. This drops the cost of lens construction dramatically!
Mirror lenses typically just have "t-mounts" and you have to buy a T-adapter for your camera. As mentioned previously... you cannot adjust the focal ratio and the lens is manual focus only. There is no communication whatsoever between camera and lens (the camera behaves as if it has no lens attached.)
The out-of-focus areas of the image will blur based on the shape of the aperture opening... which for a mirror lens is actually donut-shaped. This means out of focus areas have black spots in the middle -- resulting in a very unpleasant quality in the out-of-focus areas.
An 800mm mirror lens might cost $200 (more or less) and you'd need the T-adapter (perhaps another $15-30 depending on who makes it.) An 800mm lens actually designed for an EOS mount (with autofocus, aperture control, etc.) would cost substantially more (likely thousands).
Due to the inability to change focal ratios, the manual focus constratint, and the lack of auto-focus... there is a considerable handicap in terms of shooting speed and flexibility. You would not likely want to use one as an every-day lens.
Tim Campbell
5D III, 5D IV, 60Da
