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[OM] Re: F-stops [Was:

Subject: [OM] Re: F-stops [Was:
From: Fernando Gonzalez Gentile <fgnzalez@xxxxxxxxxxxxx>
Date: Sat, 15 Jan 2005 10:41:37 -0200
on 14/01/2005 02:31, Jan Steinman wrote:

> Actually, it's the ratio of focal length to focal width, which is
> measured at the front nodal point.

on 15/01/2005 02:04, Jan Steinman wrote:

>> why believe him over anyone else?
> 
> He makes some fairly minor technical mistakes, like fudging on where
> the focal width is measured.

on 14/01/2005 15:15, Jeff Keller wrote:

> 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.

Should I understand that the front nodal point is somewhere in front of the
lens? How does one find it? If so, I assume it is a virtual point, since
rays coming out of the front would be parallel. But if rays come out
parallel, does it make practical sense measuring the focal width at a
certain point in front of the lens?

Besides, it seems this topic has branched into two: 1- the special problem
posed by Zuiko lense, whose max aperture seems not to be at a one stop
difference from the following next; 2- the general problem of what is the f
stop, how to measure it, the implied variables, etc.

I'd open my old physics book once more, I might find some enlightment
there..., perhaps a too basic one. It's Sears-Zemansky's 'General Physics'.
I used to understand it a few decades ago.

Socratically yours,

Fernando.


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