Winsor,
Optics are truly reversible. Rheinhold's experiment and conclusions are not
contradicted by your experiment.
Gary Edwards
----- Original Message -----
From: "Winsor Crosby" <wincros@xxxxxxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Wednesday, September 18, 2002 9:06 PM
Subject: [OM] re: lens hoods/suitablity/shapes
> >
>
> That is very interesting. Thanks for sharing, Reinhold.
>
> I do wonder if your teacher was correct though. I admit that I am a
> complete dunce about optics. However I wonder whether shining light
> through the back of the lens is really highly related to light coming
> through it from the direction it was designed for. I was always the
> kid that looked through the wrong end of the binoculars and said,
> "Oooh. Everything is so small."
>
> The shapes of the projections you mentioned were thought provoking,
> especially the round projection for a telephoto which would seem to
> indicate that a rectangular cross section would not be appropriate
> for a hood. Out of curiosity I took an OM 135/2.8, extended the
> built in hood, opened the aperture all the way, and focused on
> infinity. I then cut a mask with a rectangular opening whose corners
> would rest on the front of the hood. Looking at an evenly lit
> twilight sky I could detect no vignetting on the focussing screen,
> nor could I detect any difference alternating between masked and
> unmasked.
>
> So it would appear that no light to the film plane is blocked by a
> rectangular mask in spite of the fact that it would appear that the
> opposite would be true when projecting light through the back of the
> lens. Then you wonder whether the rest of it holds water.
>
> Just thinking out loud.
>
> Winsor
> --
> Winsor Crosby
> Long Beach, California
>
>
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