Another important con: The main image issue with a "mirror lens" or
catadioptric lens is the annular or "doughnut" shaped iris-blur and
bokeh generated by the lens for out of focus highlights.

e.g see these images

http://en.wikipedia.org/wiki/File:Heron_and_donut_bokeh_crop_20070607.jpg
http://en.wikipedia.org/wiki/File:Donut_bokeh.jpg

and compare to this one from a "normal" lens (though you can see the
almost octagonal iris diaphragm

http://en.wikipedia.org/wiki/File:Bokeh_Example.jpg

The shape is essentially the shape of the entrace pupil of the lens
(an annulus ... a disk with a central circular obstruction).

http://en.wikipedia.org/wiki/File:Catadioptric_system_bokeh_christmas_tree_lights.jpg

http://en.wikipedia.org/wiki/Catadioptric
http://en.wikipedia.org/wiki/Bokeh

Not great for publication but if you just want to document a rare bird
it can work well.

The same issue occurs in catadioptric spotter scopes.

On Jun 26, 2009, at 7:02 PM, Jim Greaves wrote:

> CONS: hard to focus quickly due to not having good low light
> transmission (but, best used at dawn or dusk with sun at your back,
> on sunny days, when air is crisp and clean), sometimes a heavier
> lens actually balances better, may not be as well-constructed, but
> I've had not complaints with my Tamron (owned for nearly 30 years
> and I still use it).

--
Kevin Purcell
kevinpurcell at pobox.com