Comments about Pro Optic Pro-Optic 500mm f/6.3 Mirror Lens for Canon Manual SLR Cameras:

Excellent value at the price. However, for ~$500 you could get a Canon 75-300 IS with a noticeable increase in sharpness and not a lot more weight.

The setup: Canon 7D on Tripod bright sun, F 6.3 at 1/800 shutter speed using aperture preferred mode. Photographed objects were at 15+ meters.
JPG large output (18 MP)

Pro Optic image was entirely acceptable. Image from EOS 75-300 zoomed to 300mm and cropped to 500mm equivalent was sharper. Note that the sensor crop on a 7D is 1.6 so I'm actually comparing images equivalent to 800mm on 35mm film for the ProOptic and 480mm equivalent on the Canon 75-300 IS)

Even at shutter speeds of 1/500 and faster, you don't hand hold either of these to look at sharpness. Manual focus on the 7D with the ProOptic was slow and iffy but more the fault of using a mat screen at 800mm than anything about the lens.

One other item of note: the ProOptic produced cooler images than the Canon. Either's image could be corrected with Photoshop to match the other.

For me the only benefit to the ProOptic (because I already have the 75-300) is that I can get the full 18 MP at 800mm equivalent angle of view. Cropping the 75-300 to get 800mm equivalent gives 11MP if not resampled. Big Deal? - some times.

Comments about Pro Optic Pro-Optic 500mm f/6.3 Mirror Lens for Canon Manual SLR Cameras:

From the blurb on the product description: "Works with auto focus cameras in manual focus mode only" - and that's about it! No aperture to change so your depth of field is fixed and it's fixed at zip! accurate focusing is a must so shallow is the DoF. I tried to use this lens with my Canon 2x converter and then a Vivitar 1.4x to take a big moon shot and both times the camera failed to recognise that a lens was actually attached. But ... for [$], what was I expecting?!