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Re: [OM] Mirror lenses

Subject: Re: [OM] Mirror lenses
From: Jim Brokaw <jbrokaw@xxxxxxxxxxx>
Date: Sat, 14 Jul 2001 15:37:31 -0700
on 7/14/01 12:50 AM, andrew fildes at afildes@xxxxxxxxxxxxxx wrote:

> Thanks for the comments. 'Lower contrast' is probably good news, seeing as
> it's hard to imagine actually using it in anything but hard light. Should
> knock down the doughnuts a bit!
> I picked up a bunch of 30.5mm filters for it but notice that there is a
> Tamron 'Normal' filter with it which appears to have no effect - like a
> protection filter. Seems odd as it's not protecting a delicate front
> element but I have a very vague memory of reading that mirror lenses
> 'prefer' optically to have a filter in place so that a 'no-change' filter
> should be used when you aren't using a neutral density or other type. I
> know that Nikon make a 39mm NC filter for the back carrier of their big
> lenses too so it sounds plausible.  Anyone know if this is true?
> AndrewF
> 
I've heard that too, that the mirror lenses were designed to use back-end
filters because the front lens filters would be large and therefore
expensive. With better mirror lenses, the optics were optimized for having a
piece of glass at that location, so if you didn't use an ND filter to reduce
light transmission you needed the equivalent of a 1A filter in there to keep
the optics sharp. I don't know if the little Celestron is designed this way,
it does have a threaded back element and it came with a little ND2x filter.
With a 5.6 aperture, you need the ND2x with 400-speed film on bright days.
-- 

Jim Brokaw
OM-1's, -2's, -4's, (no -3's yet) and no OM-oney... 


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