[Leica] Light has to travel in straight lines.

[henningw at archiphoto.com (Henning Wulff)]

At 1:51 PM +0100 6/17/04, Beddoe, Neil wrote:
>The relationship between perspective and viewpoint gets 'em every time.
>
>You are standing 2 metres from a wall covered in 0.1m diameter circles
>looking through a 21mm lens at the wall. The film plane is absolutely
>parallel to the wall and the lens is 100% distortion free.  The angle of
>view of a 21mm lens is 92 degrees.  Which means you can see 4.1421m of the
>wall across the diagonal.  Assuming the centre of the furthest circle is 4m
>off centre, the near edge will be 4.38m away and the furthest edge will be
>4.56 m away from the optical centre of the lens. A fixed length of line on
>the circle's circumference at the far point will appear on the film to be
>4.38/4.56 or 96% of the same arc at the near point.?
>
>Casual observers will note that on most circles 1 degree of arc on one side
>is the same length as 1 degree on the other.
>
>Or:
>
>Buy the best 21mm lens you can get and stand 3 people up close but in a line
>parallel to the film so that one's face is in the middle and the other two
>are in the corner.  The one in the middle will have a huge nose but the
>outline shape of his or her head will be normal.  The ones at the corner
>will look as if they haven't finished teleporting from the deck of the
>enterprise.
>
>The only way to get all the circles on your lovely ornate wall to render as
>circles is to shoot from so far away that the ratio of their distances from
>the film plane approaches 1.
>
>With thanks to Pythagoras.
>
>I've really got to do some work now.
>
>Kyle, I shot five rolls of Provia recently so I feel I've earned this.
>
>Neil
>

Neil, you're confusing a number of topics.

The main point of confusion is the difference between 3 dimensional 
and 2 dimensional space. Please read my post on this topic from 
yesterday.

Anything painted on a flat wall, and imaged onto flat film that is 
parallel to that wall by a lens that has no linear distortion will be 
imaged correctly. It doesn't matter what the angle of view. Perfect 
circles in the corner of the view will be imaged as perfect circles 
on film.

On the other hand, objects with depth will not be imaged without some 
kind of distortion; here we see the exaggerated nose that you speak 
of (since the nose is significantly closer to the lens than the rest 
of the face) and globes in the corners are imaged as ovals.

-- 
    *            Henning J. Wulff
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