usher99@xxxxxxx wrote:
> Hi,
> Found this on Pnet (1999) about the Orion Mak 500. Perhaps it really is F8.
> Perhaps someone knows how to calculate the true aperture and thus DOF.
>
Sure, it's simple geometry/math. Area = Pi*R^2. Apply this to the
overall open front element. Apply again to central obstruction.
Subtract. Apply the formula in reverse to determine the diameter which
would have that area. Divide that number into the focal length.
Voila, effective f-stop.
More to the point, perhaps, at least as regards photographic lenses, I
did this to several lenses, Tamron 500/8, Tamron 350/5.6, Sigma 600/8
and Meade mirror scope marked as 1000/11.
All the photographic lenses had effective f-stops quite close to their
marked f-stops, so the correction is already made. This makes sense, as
most of these were designed when much metering was still not TTL, and
inaccurate marked stops would cause trouble.
The Meade was already less than f11 based on the full aperture I
measured - f11.9. The mirror consumed a lot smaller % of the FE area
than on the shorter lenses and the calculated effective f-stop was
f12.7. That's still less than 1/2 a stop less than marked.
So I concluded that this is a non issue in general.
Moose
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
|