I disagree with nearly everything you say. My 700/8 Questar is a very very fine lens, although not easy to use because of its focal length. Hard to aim, demands absolute steadiness and, when shot at any distance, clear and steady air. Shot with flash at its near focusing distance, it is as good as a 55 MicroNikkor at f/8 at 1:4 shot with flash. And it t/stops at t/11, not the t/9.5 you predict.PhotoJim said:The effective aperture (t-stop) is always worse than the actual optical aperture (f-stop) because the mirror is in the way, blocking some light. Expect a loss of about 1/3 to 1/2 a stop compared to a native f/8 lens (assuming an f/8 cat lens; I had a 400/5.6 Sigma once so you will see f/5.6 occasionally).
These lenses aren't great but can be good for certain types of photography. Tamron's is very good, probably the best for the money. The Phoenix I had for awhile was awful, frankly. My Sigma 400/5.6 was a good one but I had the system it fit stolen, so there was no point in keeping it. If I saw another in Nikon mount, it'd tempt me.
Dan Fromm said:I disagree with nearly everything you say. My 700/8 Questar is a very very fine lens, although not easy to use because of its focal length. Hard to aim, demands absolute steadiness and, when shot at any distance, clear and steady air. Shot with flash at its near focusing distance, it is as good as a 55 MicroNikkor at f/8 at 1:4 shot with flash. And it t/stops at t/11, not the t/9.5 you predict.
There are worse mirror lenses. Nearly all of them. And there are unusable dogs of mirror lenses. Celestron C-90, for example.
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