[Leica] laws of optics...

[jean.louchet at inria.fr (Jean Louchet)]

On Thu, 17 Jun 2004, Beddoe, Neil wrote:

>> "Second example, let's imagine a vertical wall with lots of circles
>> painted on it (let's call it the Vasarely wall).If you take a picture
>> with the lens axis perpendicular to the wall, then ALL the circles will
>> print as circles on the picture. If the camera is oblique, then all the
>> circles will show as ellipses."
> 
> Not quite true.  With anything other than a long telephoto, you'd have to
> get close to the wall and this would mean that you'd be viewing the outer
> circles from an oblique angle which would render them as ellipses in your
> photograph.  This has nothing to do with lens distortion and is just the
> normal effect of perspective. 
> 
> Neil
> 
Sorry Neil, this is wrong. I could have said in simpler terms that as long 
as the object plane is parallel to the film plane, the picture will be an 
exact (non distorted) scale reproduction of the object, without any 
distortion. This is obviously true with a pinhole camera and remains true 
with all "homographic" lenses, even wide angle. Thus if the lens axis s 
perpendicular to the wall, all circle (even far from the lens axis) will 
project as circles, not ellipses.

> Not quite true.  With anything other than a long telephoto, you'd have to
> get close to the wall and this would mean that you'd be viewing the outer
> circles from an oblique angle which would render them as ellipses in your

There is a confusion here about what you define as an "oblique angle".  
As long as the lens axis remains perpendicular to the wall's plane all the
circles will be seen as circles, even if your eye has to turn obliquely to
see it. Then, if you turn the camera then the camera axis is oblique and
the circles project themselves as ellipses.  What only counts is lens
orientation.

Jean