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Old 20-12-2007, 08:51 PM
Outbackmanyep's Avatar
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Outbackmanyep is offline
Join Date: Oct 2006
Location: Walcha , NSW
Posts: 1,652
Inverse Square law have something to do with it??

To get f/9 @ 600mm you need a wider aperture to have same light gathering ability as a 200mm f/9, but at a higher magnification. Higher quality components are needed to make the field sharp and free of vignetting @ f/2.8.
My guess is that they use a much bigger lens so that the manufacturer can control the vignetting in the lens, much like a field stop in an eyepiece.

I'd have to look into that a bit more i think to make sure!
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