The Sigma 600mm is a unique lens in a unique lens category. There are no other fixed mount (camera specific) 600mm mirror lenses that I am aware of. If you understand the inherant optical traits of mirror lenses they can be a great "nature shooter" lens provided there's ample light and you're shooting stationary subjects.The F8 aperature of this Sigma 600 doesn't allow for dark shades to render with good contrast- it's a decent long distance lens but nowhere near as good as a prime lens.....that being said, the Sigma has excellent build quality and is a definite notch above other mirror lenses currently offered by Samyang, Vivitar, Chinon, Kalimar,Cambridge etc.
Most other mirrors are at the 500mm focal length as where the Sigma has the added benefit of reaching out to 600mm- the only real competition to this lens is the "Pro-Optic" lens which is basically a telescope 500mm f5.6 lens that is mounted to SLR bodies via a threaded t-mount adapter (the Sigma is a true dedicated lens in that you must buy the lens for the corresponding camera mount you intend to use it with)- almost all other mirrors are universal in that they mount with t-mounts and can be used with different camera bodies when the t-mount is changed.....

...The Pro Optic lens (Adorama) is an unusually bright mirror in that it's a 5.6 lens and at 500mm seems to be the best "t-mount mirror" lens going.....all mirror lenses are prone to produce circular shaped "doughnut" highlights when light is refracted (when light gets sparkled as in bristled water or shiny aluminum)- hence pro's usually refrain from using mirror lenses (ever priced a 600mm prime lens??? ~$$$$$)- the Sigma 600mm produced these "highlghts" which didn't bother me personally, but beware they will appear in many situations (usually off subject)- the lens also is typical of most mirrors in that at a fixed F8 doesn't produce high contrast images in anything but bright conditions- expect a slight "haze" in medium light conditions. Also at a fixed 600mm, you'll need shutter speeds of 1/600th/sec or faster to keep images from blurring- which again warrants fast film or high sensitivity ISO settings in a digital camera- a tripod is a must with a lens of this type. I was very happy with the Sigma 600 but also knew their would be shortcomings ......if you want the best quality mirror out there go for the Sigma lens ($379)- if you want to catch that hawk up in the trees- this is the lens that will photograph it....but for nearly 1/2 the price ($209) and at f5.6 the Pro-Optic is very much worth considering (although having a much "cheaper" feeling/build) I also have a Vivitar 500mm mirror and it in no way compares respective of both build or image quality to the Sigma unit (the t-mount as supplied isn't machined correctly for Nikon mounts- mine can move radially across the mount opening)
I'm giving the Sigma 600 an "above average" rating because I feel Sigma's optic engineers have created the best mirror lens available at 600mm- despite the negative traits all mirrors exhibit, Sigma has excelled with their version.
The comparison benchmark any product should be judged according to is that it meets or exceeds it's design/function goals....not that fact that they ((mirror lenses)) produce typically poor contrast or circular highlights as that is an inherant production trait of all mirror lenses.


(I used both lenses on a Kodak DCS 620 digital camera (Nikon F5 digital))

Recommended: Yes