Re: Rambling load all right

Kennedy,

Again thanks. I think I understand it now. I believe my confusion
was created by other sites which talked about lenses that were
telecentric in both the object and image plane. Your explanation
coupled with the explanation here:
http://www.edmundoptics.com/techsupport/DisplayArticle.cfm?articleid=261

helped clear it up. It sounds like you're talking about lenses which
are telecentric on the image side only.
Thanks again for taking the time!


Kennedy McEwen wrote:
> In article <1127960880.935146.226250@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> "winhag@xxxxxxxxx" <winhag@xxxxxxxxx> writes
> >Kennedy,
> >
> >Thanks for the explanation. The only thing that still puzzles me is
> >this:
> >If a lens is fully telecentric, wouldn't that mean that a one foot
> >ruler
> >that was let's say 2 feet from the lens and a one foot ruler that was
> >4 feet from the lens would produced the same size image on the film
> >plane
> >(assuming they are within the depth of field)?
> >Wouldn't that lead to strange looking images?
>
> Not really. Perspective is still present, but the lens is just designed
> so that the principle ray is perpendicular to the focal plane, so that
> as focus moves from one plane to another, the images that are out of
> focus don't change size when they become sharp.
>
> One way to think about this, although not particularly accurate, is that
> the telecentric lens is just a conventional lens with an additional
> group at the back which bends all of the ray bundles back towards the
> optic axis by an amount proportional to their distance from the axis.
> The inaccuracy lies in the fact that the additional group obviously has
> some optical power, since all it would be is a spherical surface element
> itself, so that has to be compensated in the main image forming part of
> the lens, but that is unnecessary to explain the basic principle. The
> ray bundles still all come to a focus at the focal plane, but they are
> all symmetrical around the perpendicular to the plane. So the image
> just looks like a normal image, with correct perspective, but things
> don't change size as they pass through focus.
>
> You have probably seen the non-telecentric effect in the past without
> thinking it was odd. Take a wide angle lens (where the effect is most
> significant) and view a wall through the finder, noting where the edges
> of the frame are on the wall. Now adjust the focus from infinity to the
> closest range. With a non-telecentric lens, the edge of the frame will
> move in relative to the wall as the lens is focussed closer. This is
> because the principle rays at the edge of the frame are not
> perpendicular to the focal plane, so when the lens moves further from
> the focal plane to focus closer the rays that previously reached the
> edge now fall outside of the image frame. A telecentric lens would not
> do this - as you focus from infinity down to the closest range the edges
> of the frame would not change position. Everything still goes in and
> out of focus and has the same perspective, but their size does not
> change as the focus changes.
> --
> Kennedy
> Yes, Socrates himself is particularly missed;
> A lovely little thinker, but a bugger when he's pissed.
> Python Philosophers (replace 'nospam' with 'kennedym' when replying)

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