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Re: [OM] Sigma 600mm F8 Mirror Lens

Subject: Re: [OM] Sigma 600mm F8 Mirror Lens
From: andrew fildes <afildes@xxxxxxxxxxxxxx>
Date: Wed, 27 Aug 2003 09:56:53 +1000
Its a mirror lens. I've yet to see a snappy aperture on a mirror lens.
SNIP
 It sounds like
something to try. I suspect a large lens cap with a large hole cut in it
could give a one stop aperture to make up for focusing errors. More
things to try...

-jeff


The usual method would be to use ND filters on the rear filter mount as anything reducing the front element would cause problems - a hole in a cap would cause vignetting and increase the proportional impact of the secondary mirror reducing contrast, etc. Sticking a larger disc on the secondary mirror at the front would reduce the light coming in and would be more effective, but again would probably degrade the image significantly. I suspect that the best of these odd options would be to make a series of Waterhouse stops - filter ring for the rear mount with a set of different size hole masks as was done before the invention of iris variable apertures. This is probably not the best position for placing fixed aperture restrictions but a cheap and simple DIY project if you've got the time and inclination and it's much easier to cut small, precise holes with clean edges.
AndrewF

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