the Beauvais family
[most of a thorough, informative, and well-written post deleted so as to
concentrate on some minor points on which I may have comments...;-]
> You probably could use close-up diopters (see discussion below) on the
> front of a mirror lens, but they'd be horribly expensive since they are so
> large (77mm for mine). And at that size, you could only get the inferior
> single element diopters, not the preferred two-element diopters offered by
> Nikon and others. The combined quality would not be very useful.
Hmmm, never thought to try this... And, it is hard to predict optical
quality before actually trying it - I found in much experimenting with
macro combinations that the unlikely was sometimes excellent,
and the commonly recommended often not very good (or useful)...
I have the impression (maybe wrong...) that Canon offers a 77mm
achromat...
> 2. Use extension tubes or bellows behind the lens. How close a lens
> focuses is determined by how far you can move it from the focal plane (the
> film). The tubes or bellows increases the lens-film distance. Since there
> is no glass in a tube or bellows, the image doesn't degrade. But there is
> light loss depending on the amount of extension. And this decreases your
> working distance.
Um, adding extensions to many lenses will result in much reduced
image quality - the lack of glass in tubes/bellows is not a good predictor
of performance, and adding an achromat and/or a converter to a lens
sometimes produces sharper macro images than using just a tube.
> 3. Put a teleconverter behind the lens. This increases the focal length
> WITHOUT changing the near focusing distance. The drawback is that you
> loose one stop of light for a 1.4x teleconverter, and two stops for a 2x.
> So, you'll have working distance, but it's more likely that the bugs might
> move during the increased exposure. A 1.4x teleconverter will be
> inherently sharper than a 2x.
At small stops, things can even out, and I have had VERY sharp results
using 2X converters with the lens set around f11. Slow it is, but a
well-placed TTL flash can help a lot and not make the subject
look artificially illuminated.
> Depth of field is unaffected by the lens design. All 500mm lenses will
> have exactly the same DOF at the same aperture.
Well, DOF is a perceptual thing, and the quality of the just-out-of-focus
areas can affect the DOF (the older-version 500mm mirror Nikkor,
for instance, appears to have more DOF than a 500mm set at f8 and
the same focus distance should have...).
> Cats (mirror lenses) are inherently slow. This was an f/8, as is the
> Nikon. And the aperture cannot be varied (except for one a LONG time ago
> from Spiratone.) So you can't stop them down to get more DOF. Speed? You
> aren't thinking of hand holding a 500mm are you? Of course, there will
> always be a wiseguy usenet poster that claims that he can handhold a 500mm,
> us mere mortals waste film if we try - at any aperture of shutter speed.
Guess I'm that wiseguy...;-) I had given up on 500mm mirrors (even on fairly substantial tripods) until I tried hand holding them - turns out that my (slower)
hand-shake, combined with my damping and a shutter speed 1/500th or
above, produced better results than I could get on a tripod (with its faster
vibration rate, though lower oscillation amplitude than hand holding...). Below
1/500th, there is no contest, but to get a sharp 500mm mirror tripod image, considerable damping and/or mass must be added to the rig when using
most shutter speeds. BTW, you may find the bug photos on my web page
interesting (under "Phun Fotoz", as I recall...), and the occasional (hand held)
mirror photos also (like the 500 + 2X of the boats, under "Commercial...",
"Travel...", then "Lake", I think... (there are so many photos I put on that site...;-).
Hope This Helps
David Ruether - http://www.fcinet.com/ruether