Aperture calculation - one more time

"f/256" <askme.TakeThisOut@my.email.pls> wrote in message
news:u19Tc.1684319$Ar.871536@twister01.bloor.is.net.cable.rogers.com...
 >
 > "jjs" <john.TakeThisOut@mychain.stafford.net> wrote in message
 > news:10hprbvgimorh3d@news.supernews.com...

 >The formula is aperture in stops = F / D where F is the focal length of
the
 >lens when focused at infinity and D is the diameter of the entrance pupil.
 >But you knew that, I am pretty sure.
  >>
  > > My lens has a 112mm diameter front lens, 30mm center (where the shutter
  > > goes) and 106mm diameter rear lens.
  >>
 > If 30mm is the maximum physical opening ("D") of the lens/shutter
assembly,
 > a quick aproximation to the maximum apertue would be given by F / 30
stops.
 > To find the actual f/stop, you would have to measure the physical opening
of
 > the lens/shutter as seen from the front of the lens (search this NG for
ways
 > to measure that), in other words, the diameter you should use in the
formula
 > is not the real physical opening but the virtual opening (physical opening
 > as seen thru the lens), after all, this is how big an opening the photons
 > rushing into the lens "see and think" they have in front. Once the
photons
 > crosss the virtual opening and they realise the actual opening is in fact
 > smaller (for instance), it is too late for them, they have been tricked


Ignoring the 'virtual opening' because I haven't found what that means yet,
that makes my lens F 2.5. I'll try to find out what that virtual opening is
all about.

I just measured a 90mm F8 Super Angulon. Middle (shutter area) diameter is
14mm front glass is 46mm. By the (incomplete) metrics of F/D that's 6.4.
Wow. That unknown virtual opening is a huge factor.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Aperture calculation - one more time 

"jjs" <john RemoveThis @mychain.stafford.net> wrote in message
news:10hqb4dd89bls8b@news.supernews.com...
 > "f/256" <askme RemoveThis @my.email.pls> wrote in message
 >
 > Ignoring the 'virtual opening' because I haven't found what that means
yet,
 > that makes my lens F 2.5. I'll try to find out what that virtual opening
is
 > all about.

The real opening is the actual physical size of the opening, when looking at
that opening thru the front side, the lens itself will have an effect on
the opening you see, that opening as seen thru the lens is what I am calling
"virtual opening", but it is actually called entrance pupil.

 > I just measured a 90mm F8 Super Angulon. Middle (shutter area) diameter is
 > 14mm front glass is 46mm. By the (incomplete) metrics of F/D that's 6.4.
 > Wow. That unknown virtual opening is a huge factor.

I just did the same measurements on my 90mm Grandagon F/6.8, the glass is
about 13.5mm at the rear of the front element, that would make it F/6.67 or
so, not bad approximation if you ask me. The "virtual opening" or -better
call it- entrance pupil, would have to be would have to be 13.2mm in order
to match the F/6.8. I very very roughly measured it and I got 13mm (don't
have a point source of light handy at this time), which gives me F/6.9, not
bad either.

I believe Richard.K method to measure the entrance pupil goes something like
this: position a point source of light at the infinity focal plane of the
lens, place a piece of translucent paper on the front of the lens, you
should get a circle of light that is the image of the aperture, measure that
aperture and you have the diameter of the entrance pupil.

Guillermo<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Aperture calculation - one more time