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Old 10-01-2016, 04:19 PM
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Rigel003 (Graeme)
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Join Date: May 2006
Location: Adelaide, South Australia
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figuring f-stop with step down rings

I'm interested in trying to stop down the 77m aperture of my Canon 70 - 200 f2.4 using step down rings rather than the internal iris diaphragm. Like these. Trying to avoid having bright stars show the diffraction pattern caused by the iris.

Is there an easy way of working out what diameter ring you'd need the aperture to be to create the various f-stops? For example what diameter ring would create the equivalent of f/4 if I'm using the lens at 70mm or at 200mm? You'd think it would be just a simple matter of division of aperture by focal length, however 200 divided by 2.8 is not 77.

Last edited by Rigel003; 10-01-2016 at 04:51 PM.
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