I had a mirrored lens (400mm). It worked ok, but I had it for a film camera and you can't adjust the aperture. This was more of an issue on the film camera I had, but long focal length lenses have a very small depth of focus and stopping down the aperture helps that.

Canon tried a fresnel lenes there "DO" series. Not quite as good quality at about half the size and way more money. Its interesting technology.

You can see the lens size comparisons near the bottom of this article.

https://www.the-digital-picture.com/Reviews/Canon-EF-400mm-f...

One of the main things driving larger optics/ is the amount of light the lens lets in. EG: max aperture (a lower f number) means bigger lens, more money. The availability of digital sensors means often you can shoot with higher film speed and might not require as large a max aperture..

I picked up a Tamron Adaptall 55B 500mm f8 two and a half years ago. They were produced from 1979-1983 and sold new for about $400 as best I can tell.

The big difference from contemporary Samyang and Rokkinon catadioptrics is it's really long focus throw. It's about 350 degrees from 1.5m to infinity. At 500mm even f8 is shallow. Hyper-focal distance is over a kilometer on a crop sensor.

Even with a 2x teleconverter (1000mm f16/t22) it's less than a kilogram so it's easy to walk around hand holding and throwing it in the bag is not a major commitment.

Like any other lens it's a compromise. I live with it's technical shortcomings because it provides a specific capability to make images I would not make otherwise. In theory, it's a choice between lenses based on technical specifications. In practice it's a choice between making image with the catadioptric and no image at all. The images that would not exist otherwise exist because I can see in 500mm prime before I raise the camera and when my eye hits the viewfinder I'm not making a "well maybe I can crop it" compromise.