>
> > I bought a Celestron 300/5.6 mirror lens because it is very compact. A
> > T-mount, but since it doesn't change aperture anyway this is fine.
> Realizing
> > that the 5.6 aperture is likely to be dim I got a #1-7 screen, which is
> for
> > telephoto and slow lenses. I put this screen in my OM-4 to use at a
> > motorcycle race with the 300...
> >
> > With the 300 mirror lens, the viewfinder is almost unusable. The image is
> > severely vignetted, resulting in a circular image about 1/3 of the screen
> > width only in the middle of the finder. The microprisms are blacked out,
> and
> > the fresnel is very visible.
> >
> > I tried the 1-7 screen in the OM-4 with my Olympus 300/4.5 lens, and it
>
Wonder if this could have anything to do with the large secondary obstruction
a F5.6 Catadioptric lens has???
Mark Hammons
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