I am considering buying a manual focus 800mm f/8 mirror lens to use on birds and birds in flight. I have been using mostly manual focus lately anyway--even on flying birds. The lens would be longer and faster than my 500mm and a 1.4x teleconverter. I have seen about four brands, Vivitar, Rokinon, Bower, and Phoenix. There is not a lot to choose when it comes to price. I want to buy the sharpest one I can get. Does anyone have any experience with these lenses and what lines per millimeter they deliver?
Not sure what you mean by lines per millimeter Paul. I don't think that is a function of a lens.
I would look for images created with these lenses before you get one. You may not be happy with the image quality compared to what you are now using.
Some are good, but I've never seen one with auto focus, and the fixed aperture will be very limiting.
There are two pips in a beaut,
four beauts in a lulu,
eight lulus in doozy,
and sixteen doozies in a humdinger.
Nobody knows how many humdingers are in a lollapalooza.
I believe that all four brands are actually the same lens. Mirror lenses produce donut shaped out of focus background images that a lot of photographers don't like. They also have only one aperture. Other than that they are good inexpensive long teles.
Some of the best quality mirror lenses were made in Russia of all things. I have a 1000mm f/10 which is capable of extremely sharp images but in a lot of situations it is nearly useless. It is really hard to focus with that 1/10 aperture and it also requires a really solid tripod or you will have a lot of shake unless you are able to use a fast shutter speed and usually you can't.
They also are easily damaged. I have an older Sigma 600mm which is also pretty good. Also a Tokina and a Minolta.
All those made now as far as I know are like someone else said, basically the same lens. They are sharp enough for most uses and extremely handy. I did once have one which was so bad that I could tell it wasn't sharp just by looking through the viewfinder.
One thing about them which is pretty neat is if you have an adapter to convert a lens to a telescope, these work pretty well and are really powerful.
I believe I had the same f/10 1000mm you did years ago. It was made in Russia for Cambron. The lens was very sharp. The problem I had with it was that one of the elements extended inside the body of my Nikon FG20 and my mirror began to hang up on the rear most part of the lens. I wish I could have realized that a small extension tube would have probably corrected the problem. As it was, Cambron replace the lens with one that was not very sharp at all.