From: jacobson@cello.hpl.hp.com (David Jacobson)
Subject: Re: Filter/Optics Question...
Date: Mon, 21 Dec 1992 01:50:00 GMT
Organization: Hewlett-Packard Laboratories

In article <1992Dec18.021315.12595@adobe.com> wtyler@adobe.com (William Tyler) writes:
>Of course a wider angle lens needs a bigger filter ring, all else
>being equal. However David's comment about the periphery [I had said
>that vignetting results from the filter ring obstructing the perphery
>of the lens---dmj] of the lens
>reinforces my point about stopping down. Here's why. If you think
>about imaging a point near the edge of the field of view, when you've
>got a too-small filter on, the part of the lens that is near the point
>angularly is shaded, but the other part isn't. Consider the diagram
>below. 
>
>                   *
>	          /
>	         /
>	        /
>	       /
>       _______/
>       \     /
>	\   /
>	 | /
>	 |/
>	/
>       /______
>
>It should be clear that your only chance of imaging the * at all is if
>you are using the periphery of the lens on the side of the lens
>opposite the *. Stopping down prevents this, thus the vignetting is
>much more visible when stopped down.
>
>Bill

The usual situation when a filter ring causes vignetting is depected
in the following diagram, which is the view through the upper left
corner of the frame.  In the diagram #### is the ideal full aperture,
***** is the edge of the filter ring, and ... is the size of some
smaller aperture.  As can be seen, at full aperture, the lower right
portion is blocked, reducing the total area through which light can
pass, and reulting in vignetting.  But when one stops down the the
smaller aperture, as indicated by .... , this smaller aperture is free
of obstruction and there is no vignetting.

           ############
         ##            ####
       ##                  #
     ##                     ##
     #                      * #
    #                      *  #
    #          ......     **   #
    #          .    .    *     #
    #          ......  **      #
    #                 *       #
     #              **       ##
     ##           **        ##
       ##   ******         #
         ##*           ####
           ############

In the case Bill describes, the view seen through a corner of the
frame would have to look like this:


            ###########
          ##      *    ##
        ##       *       ##
      ##       **          #
      #      **             #
     #    ***  .....         #
     #  **     .   .         #
     #**       .....         #
      #                     #
       #                   ##
        ##               ##
          ##           ##
            ###########


and indeed, these is some light reaching the corner, but stopping down
makes it go completely black.  But note that even with a wide open
aperture there is a minimum of 1 stop of vigentting, since the
unblocked portion of the lens is not even half its area.  So there is
no hope of eliminating vignetting by opening up wide.  

I assert that this is an extreme situation, and the normal sitution is
the first one, where stopping down does eliminate vignetting.

  -- David Jacobson




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