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RE: [OM] focussing aid/screen Q

Subject: RE: [OM] focussing aid/screen Q
From: "Ian A. Nichols" <I.A.Nichols@xxxxxxxxxxxxx>
Date: Tue, 30 Nov 1999 14:00:31 +0000 (GMT)

On Tue, 30 Nov 1999, Olaf Greve wrote:

> I think this is a phenomenon inherent to any type of
> lens, where the image that is produced by rays of light coming in
> perpendicularly through the very centre of the lens (i.e. in this case: the
> ones that are projected on the centre of the focusing screen) should always
> be sharp as these rays are not "bent" by the lens, contrary to the rays
> coming in at other angles, or other parts of the lens. Correct me if I'm
> wrong here.

Sorry Olaf, I'm going to have to correct you.  Light which reaches the
centre of the focusing screen could have entered the lens at any point
on the surface of the front element.  Consider a ray of light from an
object in the centre of the field of view:  if the ray goes through the
centre of the lens, it will not be deflected and will hit the centre of
the screen; if it goes to the edge of the lens, the lens will deflect it
so that it still winds up hitting the centre of the screen.

You do (of course) get a falling off of intensity towards the edges of
the image, but this is because the rays of light which form the image
have, in effect, a much smaller area of glass to fall on since they are
approaching from the side rather than the front.

A simplistic explanation, but I hope an intuitive one.

-- 
________________________________________________________________________
*             |                                                        |
|  /  | |/-\  |                      Ian A. Nichols                    |
| |   | |   | |                                                        |
|  \-/| |  /  |                  i.a.nichols@xxxxxxxxxx                |
|             *                    iann@xxxxxxxxxxxxx                  |
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