Allen Coltrin wrote:
> ....
>
> So a question please. If I'm looking for a great 500mm+ mirror telephoto
> lense what would you guys recommend and why. I'm pulled to the Oly, Tamron,
> or Tokina. I bough to the weighty experience and judgement of this esteemed
> body.
>
Well, I see you have seen Ian's experience with the E-500. I don't know
if this is common. I don't see why it would happen only to Oly bodies as
a matter of lens/body incompatibility. I can see how there could be a
problem focusing them on small sensor bodies in general and the E-300
and 500 in particular. These lenses are slow and those bodies aren't
designed with MF capability in mind. Accurate focus could be difficult
without a magnifying finder and/or a lot of light.
In the test shots I just made, even with a FF body, tripod mount,
stationary subject and 2x finder magnification, precise focus took some
care.
Here is a summary of the tests Modern Photography did on many mirror
lenses <http://brashear.phys.appstate.edu/lhawkins/photo/mp-cat-tests.txt>.
Contrast isn't quite the issue it was back then, before the digital
darkroom.
There are several issues that aren't immediately obvious. First,
performance:
- No mirror lenses I know of have adjustable apertures. There has been
discussion here of how it might be done, but I don't know of practical
applications. That means you are stuck with the wide open aperture. The
minor effect is in bright light. With film cameras, the inability to
change sensitivity on the fly and the relatively long highest shutter
speeds can lead to the need for a neutral density filter. No problem for
DSLRs in general. More serious is the shallow depth of field. With
conventional lenses, you can stop them down to get greater DOF.
Chuck has made some comments on DOF calculations. They dont' square with
my experience with mirror lenses. The issue is that most DOF calculation
assume a relatively modest magnification for viewing. With mirror
lenses, I find myself quite often cropping and blowing them up a lot
more. That results in shallower effective DOF. You can see the effect
here, as I zoom in from full frame to a close-up over three images. By
the time you get up close, DOF is no more than an inch or so
<http://moosemystic.net/Gallery/MPhotos/Maine/Sigma600/pages/05_1682e.htm>.
- Mirror lenses give funny looking out of focus highlights. You can see
a mild case on the right side of this image
<http://www.moosemystic.net/Gallery/tech/Lenses/Sigma600/Juniper.htm>.
Much more obvious here
<http://www.moosemystic.net/Gallery/MPhotos/Maine/Sigma600/pages/12_1695lces.htm>.
It can also lead to other funny OOF effects
<http://galleries.moosemystic.net/Garden04/pages/3649_16.htm>. This
drives some folks crazy, and is acceptable to others.
- Mirror lenses generally have lower contrast, and often lower resolving
power, than conventional lenses of the same focal length. The trade off
for their small size and weight.
There are also specific issues with filter capability of different
makes. The Zuiko 500/8 requires large, expensive 72 mm filters. Most
others use small filters that screw into the back of the lens. Light and
cheap, but changing the filter requires un-mounting the lens. Especially
problematic with many DSLRs. The Sigma is unique, with a holder for
small filters that slips into a hole near the back of the lens.
Another thing to consider, for all long lenses, is how they will be
used. I often find myself wishing for either a longer lens or a way to
get closer to the subject, resulting in cropping the result. In this
situation, measured lens resolving power becomes relative, not absolute.
If a 500 and a 600 mm lens both have the same measured resolving
capabilities, shots of the same subject from the same distance will
resolve more subject detail with the 600 mm lens, as it's image doesn't
have to be enlarged as much for the same display size.
That, build quality and the superior filter design are the reason I've
considered the Sigma 600/8 generally a better choice than any of the big
crop of 500 mm mirror lenses.
When all is said and done, I would get at least a good 300mm prime lens
before playing with mirrors. My preliminary test results show the Zuiko
300/4.5 with 2xA outperforming the Sigma 600/8. There are also some big,
long zooms that will outperform mirrors for many purposes. The Tamron
200-500 and Tokina 150-500 come to mind
<http://moosemystic.net/Gallery/MPhotos/BayArea/Delta/Falling.htm>.
Moose
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