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.
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..
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..