William writes:
>All mirror lenses are spec'd with their f/stop, because it looks better
>in the ads. The T-stop is about a third-stop smaller (depending on the
>size of the secondary obstruction).
You are right, I have just looked for it. The diameter of the front element
with 500mm and 1:5,6 should be circa 90 mm with an lens without central
obstruction. With the Rubinar and its obstruction, the diameter is also
circa 90 mm, so you have to substract the central obstruction to calculate
the exact effective aperture. The central obstruction has a diameter of 50
mm, so the obstruction is 17 percent, if I am right.
Matthias
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