[Leica] Light has to travel in straight lines.

[dpost at triad.rr.com (Dan Post)]

Neil-
In re your "light has to travel in straight lines...."
Now you done gone and done it! Unraveled 30 years of misconception and
quasi-pseudo science!
It took me almost a full semester to understand enough physics to comprehend
that light could be bent, either by diffraction or in the case of Einstein's
theory, by exceptionally massive objects!
By the end of that semester, I had one nerve left, composed of no more than
a dozen very frazzled neurons, but I thought I had gained some ground. Oh,
utter woe, how wrong I was! :o)  I never managed enough courage to take a
course in optics!

Seriously, the distortion you mention is not entirely a characteristic of
either the 21SA or wide angled lenses in general! I remember in my younger
days, I tried to impress a young lady named Fred (Winifred, actually) by
doing a 3/4 close up head shot of her with the only lens I had at the time,
a 50mm Mamiya-Sekor!
I never showed her the photo! Even the 50 made her ear look about half again
as large as her nose, and despite her inherent beauty, it made her look,
well... funny! Almost like a Fun-house Mirror. I've come to appreciate my
1932 vintage 9cm Elmar for ladies' portraits!

Dan



----- 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 8:51 AM
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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