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[OM] Re: f-stops (Was: Solstice web-site news)

Subject: [OM] Re: f-stops (Was: Solstice web-site news)
From: "Jeff Keller" <jrk_om@xxxxxxxxxxx>
Date: Mon, 17 Jan 2005 10:23:37 -0800
I've seen the NA figure for microscope lenses but have never dug into where
it came from.

You also switched from sin to tan to calculate the f-stop [which makes sense
to me ;^) ]. The light striking the film plane has to act similar to a
simple lens (?) whether it is a retro-focus or telephoto ... but what was
that Olympus said about the advantages of retro-focus for digital cameras
..

Clearly I'm confused about the cone of light on the back side of the lens
.. probably the difference between off axis angle and angle due to edge of
aperture? I guess I need to look at the 4/3 explanation again.

Any short explanations for NA using sin and/or not using f-stop for
microscope lenses?

Thanks,
-jeff

----- Original Message -----
From: "Joe Gwinn" <joegwinn@xxxxxxxxxxx>
Subject: [OM] Re: f-stops (Was: Solstice web-site news)


>
> The NA is n sin(a), where "n" is the refractive index of the
> image-space medium and "a" is the half-angle of the converging code
> of light.   For a photographic lens, the medium is air and so
> n=1.0000.
>
>
> The half-angle "a" can be computed from the f-stop:
> Tan(a)=(diameter/2)/(focal length).  For our purposes, the NA isn't
> needed.
>
> So, measure the diameter of the converging cone of light just after
> the last lens, and also the distance from the plane in which that
> diameter was measured to the image plane (where that cone ends in a
> point), and compute the f-stop from that: fstop= distance/diameter.
> It isn't necessary to actually compute the half-angle.
>


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