[Leica] laws of optics...

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

Hi,

No it does not repel the laws of optics. I'll try with two examples.

Let's say the moon (on a "full moon" day). When one looks at it, it is a 
circle. On a picture, it will onlyu be a circle if it is in the centre of 
the picture (i.e. when the lens is aiming at it, or when it is in the 
symmetry axis of the lens). Otherwise it will appear as an ellipse on the 
picture.

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.

All the above is only true with geometrically "perfect" lenses, and most
Leica lenses are very, very near to geometrical perfection.

The question is, what is the definition of "something circular"? The
Vasarely circles are perfect circles in themselves, but a circle is not a
volume, it is a planar figure, and the way it will appear on the photo
depends on whether the lens axis is perpendicular to this plane or not.

Ordinary cheap lenses are usually unable to render correctly the "vasarely
wall" because of their distortion (specially cheap zooms and cheap wide
angles).

For math-oriented luggers, one can say that the geometric transform made
by a good lens is a homography, and with a cheap lens it is not a
homography but something much more complex. Homographies always show
straight lines as straight lines. On a purely geometrical point of view,
the old-fashioned pihole is a perfect lens.

Fish-eyes are another story, as they do not try at all to keep straight
line straight. The problem with homographic lenses is that the angle can't
approach 180 degrees (the focal distance should be near zero to get close
of 180 degrees), and in practice the shortest focal lengths (in 24x36mm)
which are still homographic are around 12mm. In order to access extreme
wide angles it is necessary to get rid of the homographic constraint, and
to accept that straight lines that do not cross the centre of the image 
show as curved lines: this is a fish-eye. 

This was the geometry recreation :-)

I don't write very often on the list but I appreciate its rare mix of
technical artistic and human, very human topics. For those who don't know
me (all?) I am an amateur photographer, doing essentially stage
photography (theatre, concerts), portraits, landscapes, and some related
to my main hobbies: musical performance and instrument making. On the
professional side I have been involved with digital image processing since
1977 (but personnally still sticking to film).

Best wishes

Jean


> Don,
>
> The 21 SA repeals the laws of optics?  Something circular in the corner
> of the frame does not become an oval with the 21 SA?  Please post a
> picture to show this miracle.
>
> Bob
>>
>> As you already know, the 21 SA has just about zero distortion: you can
>> put something in the corner and it will look normal.  One of the
>> reasons I put up with the relative slowness and the lack of metering
>> with an M is the absolute fabulous performance of this wide angle.
>>
>> Don