Hybrid use query about equipment purchase

"jjs" wrote in message
...

"Bob Monaghan" wrote in message
...

[... it's about mirror lenses ...] You get the worst of both worlds, a
lens delivering
only f/11 amount of light, but with fixed DOF of an f/8 rather than the
greater f/11 DOF you might want ;-)

If speed isn't an issue, you _can_ have variable DOF with a mirror lens.
Just cover the front of the lens and place an aperture to one side of the
centre mirror.


Would a spare lens cap with a large hole in it, so it covered just part of
the edge of the front mirror all the way around, be an equally good
solution? A disk in the centre that was a bit bigger than the 'centre plug'
ought to work too, but would be less convenient to attach.


Peter

ah, no, probably not. I have a 25cm telescope, and the manual also
recommends an off-side circular/ellipsoidal cutouts. This is very
different from a ring for various reasons. In effect, the circular
off-center cutout is acting to create a smaller diameter circular mirror,
hence the smaller f/stop equivalence. There was an article a number of
years ago in Modern Photography, IIRC, by Herb Keppler showing a series of
cutout layouts for various f/stops. You cut them out with cardboard,
though a cutout lens cap would probably work. Some of the astro sites may
have more info.

here is a note by lens designer Brian C. on this from posting at
http://medfmt.8k.com/mf/mirror.html

From: (brian)
Newsgroups: rec.photo.equipment.medium-format
Subject: MIRROR LENS Special problems
Date: 21 Jan 2003

....

Bob:
When you put a cutout in front of a catadioptric lens the cutout
*becomes* the aperture stop, and the DOF is increased accordingly.
Essentially what you are doing is creating an unobscured system with a
front stop. The main problem when you do this with a 500/8 is that
the f/# gets really large, which will cut down the resolution. If you
want to do this trick a 500/5 would be a better choice to start with.

Brian
www.caldwellphotographic.com
====

endquote

hth bobm
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