[Leica] Light has to travel in straight lines.

[nbeddoe at lehman.com (Beddoe, Neil)]

Yes,

In a ridiculously convoluted way, that's what I said.

-----Original Message-----
From: animal [mailto:s.jessurun95@chello.nl] 
Sent: Thursday, June 17, 2004 1:58 PM
To: Leica Users Group
Subject: Re: [Leica] Light has to travel in straight lines.


sorry i don,t quite understand.wouldn,t the outer circles be rendered narrow
iso of smeared out?
simon
----- Original Message ----- 
From: "Beddoe, Neil" <nbeddoe@lehman.com>
To: "'Jean Louchet'" <jean.louchet@inria.fr>
Cc: "'Leica Users Group'" <lug@leica-users.org>
Sent: Thursday, June 17, 2004 2:51 PM
Subject: [Leica] Light has to travel in straight lines.


> 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
>
>
> -----Original Message-----
> From: Jean Louchet [mailto:jean.louchet@inria.fr]
> Sent: Thursday, June 17, 2004 12:53 PM
> To: Beddoe, Neil
> Cc: 'Leica Users Group'
> Subject: RE: [Leica] laws of optics...
>
>
> 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
>
>
>
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