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[OM] Re: 750mm f6 Catadioptic

Subject: [OM] Re: 750mm f6 Catadioptic
From: "Jeff Keller" <jeffreyrkeller@xxxxxxxxx>
Date: Sat, 13 Jan 2007 15:12:42 -0800
YMMV but when I'm using a long lens it is most often trying to capture
a bird or other creature. The DOF of the CAT lenses is plenty if I can
focus it. I've also used the 750/6 to pick off a distant light house
and a power plant. DOF was not a problem for either of those. The
distance compression can yield nice images.

I've used my 350/5.6 and 450/4.5 much less but neither of those give
me worries about too shallow of DOF.

Sigma made a fairly long, fairly fast catadioptric lens which had a
lens cap which had a smaller lens cap in the center of it.  Awhile
back someone posted a link to a mirror lens which had a variable
aperture. The gain in DOF is much less than expected because the
center obstruction keeps the diameter of the lens large. At that time
I tried to track down one of the variable aperture mirror lenses. The
few user reviews I found rated the lens as essentially useless. I did
find someone that had one for sale. He wanted almost $300. I wasn't
that curious nor could I find a cheaper one.

-jeff

On 1/13/07, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
> Hmm.  Apart from a couple of quick shots on an OM the only thing I've
> ever used this thing for is the sky.  I hadn't stopped to think about
> the DOF problem.  I see that at 50 feet it's only about 1.1 feet.  But
> going to f/8 doesn't help much as it's still less than 1.6 feet.  Maybe
> it will be necessary to make an aperture mask for it.
>
> My only other mirror lens is my 2000mm f/10 Celestron which will focus
> down to 25 feet.  Now *there's* a DOF problem.  At 25 feet it's about 1/4".
>
> Chuck Norcutt

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