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[OM] Re: 750mm f6 Catadioptic

Subject: [OM] Re: 750mm f6 Catadioptic
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Sat, 13 Jan 2007 17:51:45 -0500
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".

Hmmm.  Don't know if I ever noticed this before but, using the DOF 
calculator just now, I suddenly realized that DOF increases by the 
square root of 2 when closing down one stop and by a factor of 2 for two 
stops.

Chuck Norcutt

Moose wrote:
> Chuck Norcutt wrote:
>> This reminds me that I have an Orion 500mm f/5.6 mirror lens/spotting 
>> scope that I've almost never used...... 
>> so maybe I'll give this lens a whirl again.  At f/5.6 I may actually be 
>> able to focus it.
>>   
> I sure hope so, because DOF is going to be 'interesting'.
> 
> I think all those 500-600/8 lenses are f8 not just because of size and 
> cost, but for DOF reasons.
> 
> Have fun and let us know, maybe even, gasp, post some samples. :-)
> 
> Moose
> 
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