I don't believe the one particular sentence
"The calculations above would be accurate if the aperture blades were
mounted right in front of the front element."
For a single element lens, its diameter could be used to calculate the
maximum aperture f stop.
With a compound lens even if the front element is as small as possible to
avoid vignetting, its diameter often wouldn't be the appropriate diameter to
calculate the maximum aperture f stop.
The simple and elegent experiment proposed earlier by Julian of placing a
point source of light at the focal plane on the lens axis and measuring the
diameter of the light coming out the front would be a good way to actually
measure the maximum aperture. If the point source is exactly at the focal
plane the light coming out the front would neither converge nor diverge. The
rays would be parallel.
The f-stop concept works because light spreads out on the camera side of the
lens with the square of the distance. A lens having the same diameter but
twice the focal length would have 1/4 the light intensity at the film. By
keeping the ratio of diameter to distance constant irrespective of focal
length, the intensity remains constant.
-jeff
----- Original Message -----
From: "Walt Wayman" <hiwayman@xxxxxxx>
Subject: [OM] Re: f-stops (Was: Solstice web-site news)
> Chris,
>
> I believe this fellow's explanation of f-stops, et al., because:
>
>
> Walt
>
> -------------- Original message ----------------------
> From: Chris Barker <ftog@xxxxxxxxxxxxxxxx>
> >
> > That's a nice handy website Walt, sure enough, (and I have bookmarked
> > the site to return later, perhaps for inspiration or information) but
> > why believe him over anyone else?
> >
> > Chris
> >
> > On 14 Jan 2005, at 05:04, Walt Wayman wrote:
> >
> > > Further sayeth I not. At least for now.
> > >
> > > http://www.uscoles.com/fstop.htm
> > >
> > > Walt
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