On Wed, 2007-05-16 at 09:31 -0700, William Chops Westfield wrote:
> On May 16, 2007, at 8:55 AM, Rich Satterlee wrote:
> 
> > within fairly tight confines of picture taking, the mirror lens is
> > no where near as good as the "big glass" of the professional folks.
> 
> Which of the things you were complaining about would have been
> better with a glass lens of equal focal length?  My general
> impression was that the mirror lenses were easier to use by virtue of 
> being lighter.  Or are you talking about being saved by higher shutter
> speeds with REALLY big glass like this 200-500mm f2.8:
> http://www.dpreview.com/news/0703/07030805sigma200500mm.asp
> (wow!)

Coming from the telescope world, mirror type setups have the advantages
of far less weight, far smaller size (depending on design, I'm assuming
most photo-graphical mirror type lenses are of either the
Schmidt-cassegrain design, or the slightly different Maksutov type lens
basically a superset of the schmidt-cassegrain design, adding a
"corrector lens" at the front to correct spherical aberation), and
correspondingly far smaller costs.

The main disadvantages to mirror type lenses are the "donut effect" (out
of focus point sources appear as donuts instead of circles) and
sensitivity to "jarring" which puts one (or both) of the mirrors out of
alignment. 

For a hobbyist neither of these is much of an issue, the donuts are
annoying, but no big deal to most hobbyists (especially considering the
amazing shots one can get). The fact the lens is more sensitive is also
not to big a hassle since many hobbyists are VERY gentle with their
equipment.

For professional photographers these issues can be huge. The donuts are
just not acceptable to most professionals, if there is another option.
The bigger issue is probably the fact that the lenses are more fragile;
when you're lugging a camera around all day every day, bumping into
people, climbing onto things, waiting in the pit stop area, etc., a
fragile lens will be out of alignment (or worse) very quickly.

TTYL



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