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Re: [OM] mirror lenses......puzzling

Subject: Re: [OM] mirror lenses......puzzling
From: Vaughan Bromfield <vaughan.bromfield@xxxxxxxxxx>
Date: Mon, 19 Mar 2001 12:18:08 +1100
So far the only observed discrepency has been with the *meter* reading
and viewfinder image, both of which may be fooled by the annular
aperture in the reflex design. The actual image may not be affected at
all: to find out, make some exposures with slide film, bracketing around
the expected and indicated exposure, and see what happens.

So go out and expose some film... please. It's the only way to
definitively end the debate.


Vaughan

--

> Date: Sat, 17 Mar 2001 12:58:59 +0100
> From: Thomas Bryhn <thomas.bryhn@xxxxxxxxxx>
> Subject: Re: [OM] mirror lenses......puzzling
> 
> At 19:24 16.03.01 -0500, John Raymond wrote:
> >  I would say with the mirror the viewfinder appears to be
> >atleast an f8 or slower.  I am puzzled.
> 
> That is puzzling. f/8 means you you loose 65-700f the light somewhere in 
> the optical path. The mirror coatings would have to be *very* bad to 
> account for that, and this thing is only like 20 years old.
> 
> >The confusion I have is that I have read from other mirror users that this
> >loss of light is common and the f stops are not accurately described on
> >mirror lenses.  I thought then I wasn't alone.
> 
> Nothing is perfect, but I would assume a lightloss about 0.3 stops, 
> certainly nowhere close to 1.5 stops. Check out the measurements at 
> http://brashear.phys.appstate.edu/lhawkins/photo/mp-cat-tests.txt
> The worst is the Vivitar 600/8 at f/8.9, and that's almost exactly 0.3 stops.
> 
> >  The only thing I can see possibly causing problem (this
> >might be huge) is that the lens I have doesn't have the UV filter on the rear
> >end.  I am not sure that it is part of the optical formula as it would sit
> >right against the last real element anyway.
> 
> A missing *flat* filter will not cause lightloss, on the contrary, but any 
> rear mounted filter should be part of the optical formula, as it otherwise 
> will cause spherical aberration. You should always leave a filter in there.
> 
> >Also, the t-mount doesn't stop in accordance to the built in-non movable
> >tripod mount.  If it were to be on a tripod I would have to tilt the platform
> >quite a ways to bring it to "flat".
> >I have heard that some t-mounts can be adjusted to get this alignment-maybe I
> >have a bum t mount as well? : )
> >I will also research this.
> 
> A T-mount adapter will usually have an inner ring that can be rotated after 
> losening 3 or 4 screws. Is it possible that your T-mount has a very small 
> opening thereby causing serious vignetting? If not, take the whole thing 
> back to the shop or have another shop look at it. You've probably paid good 
> money to have f/4.5, not f/8.
> 
> >Thanks Thomas for your thoughts and any others who have responded or will.
> 
> You're welcome, hope you can sort this out! Now, if anyone knows of another 
> fast mirror lens, a Celestron Comet Catcher, for sale, (500mm f/3.64) I'd 
> love to hear about it.....
> 
> Regards,
> Thomas Bryhn
> 
> ------------------------------
>

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