>They also tend not to be as good as refractors.
The main effect of the central mirror is the drop of contrast and
resolution -- it's because of the optical laws! If the central obstruction
is no more of 20-25 0n diameter, there's nothing to worry about, but over
30% resolution halves. Contrast also suffers, but the bright colours from
these lenses may compensate that -- if you aren't shooting B/W.
I own the Tamron SP 500/8. It makes good pictures (if you don't put an UV
filter in front of it!), but contrast is lower than other lenses. I have no
experience with the Zuiko 500, but tests show higher contrast than the
Tamron, even with about the same central obstruction (44%). Maybe the
Zuiko's coating and inner painting is much better than the Tamron's, so it
gets closer to the theoretical limit.
>I don't know why they
>don't recommend the 1-7, which is specifically for super-teles. In any
>case, both the 1-7 and the all-matte Beattie work with my 1000/11.
The 1-7 works OK with a small, _unobstructed_ aperture, like that on the
1000/11. The central obstruction in a mirror lens will darken the
microprisms, despite the larger aperture. However, the all-matte Beattie
(or the 2-4 screen) should be fine.
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