Tamron SP Adaptall-2 500mm f/8 mkII (Mirror)

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compact, light weight, approximates genuine f8, very sharp for this type of optics
razor thin DOF, donut bokeh, no diaphragm, rear screw-in filters

There are those who love mirror lenses and there are those who hate them. I'm in the former group and this lens is one of the reasons why. It's easily handholdable in terms of size and weight but nevertheless superbly built and equally sharp as the best mirror lenses produced by the major camera manufacturers of the day. By comparing it with various dioptric lenses (metering through one and the same body) my estimate is that the Adaptall-2 500 f8 really has an effective aperture close to f8, unlike many other catadioptric lenses where the f rating ignores light loss caused by the central plug in the front lens. There is no noticeable chromatic aberration even with extreme contrast.

On the other hand, being a mirror lens means that it has practically no depth of field so it requires extremely accurate focusing, and in practice you always need to give it as much support as possible (by tripod, bracing yourself against a tree or a wall etc) while focusing. Color, UV or neutral density filters have to be screwed in the back, so adding or switching filters means you have to dismount the lens first. Also, if you don't position your eye exactly at the center of the viewfinder then at f8 the split prism focus screen of your camera will often black out in half the central circle, thus becoming even harder to focus with. Because of the center plug, highlights in the background will show up as rings instead of evenly diffuse spots, so to avoid these donuts you need to carefully reposition yourself to minimize any reflections from background objects. And because light is bounced off mirrors instead of passing through multiple coated lenses, colors are flatter than they would be with a dioptric lens.

For me the benefits of small size and light weight are often much greater than the drawbacks, because a lens like this can travel with you much more easily and will attract much less attention than a big expensive dioptric 500mm lens with apo-quality glass. If light is plentiful and the need to move around precludes you from working with a big heavy lens and a suitably robust tripod, then a well-built mirror lens is the solution. Within those constraints, in my experience the SP 500 f8 is one of the best choices around.