Re: [Leica] Re: Distortion

["Henning J. Wulff" <henningw@archiphoto.com>]

Danny,

>'Oblique projection' is fully dependant, via imaging angle, on the
>co-relation of
>flange back length vs. focal length. A 20mm lens, with a symmetrical cell
>in the rear
>(identical to the front), has a nodal point at 20mm's from film. If you
>draw a
>triangle from the conjugate, "image distance" or nodal point of the lens,
>to the
>corners of the image, the coverage angle required is large with far
>travelling rays
>forming the image edges and corners; making a squat triangle.

<snip>

This and some of the subsequently quoted texts re: the angle of the rays
between the lens and the film, is not correct when referring to a
rectilinear lens which has no linear distortion, whether retrofocus or
regular construction.

Note, though that the main part of the text you quote with respect to the
perspective distortion effects seen when imaging spherical objects deals
with the rays from the subject to the lens, and this is correct.

With regard to perspective rendering, what matters is the angle of
incidence on the lens, not the angle of the rays between the film and the
lens. You can have whatever type of relay lenses you want behind the
collector lenses, and as long as you wind up with no linear distortion, you
won't change the perspective rendering. The lens - to - film angle does, as
you mentioned, affect the eveness of illumination, with the result that
retrofocus lenses don't suffer the full effect of the cos-fourth law. And,
as you quite rightly pointed out, the viewing distance/image size
relationship is critical if one wants to overcome perspective distortion
effects.

>...and so, doesn't require the image magnification at frame edge that the
>RF version would...

Variable image magnification is the definition of linear distortion. If a
lens is truly rectilinear, it does not have variable image magnification.
This is really the crux of the matter.

>...Two solutions are made in lens design: the first is the fisheye lens,
>which renders that circular perspective directly 'as is' to the film...

Fisheye lenses do _not_ render circular objects or anything else 'as is'
onto film, anymore than any other lens design does. Just different
compromises are involved. Fisheyes have their 'flavours' too; just think of
the 10/5.6 OP fisheye of Nikon's, which produces images with _no_ falloff
to the edges due to its orthographic projection formula. (Some of this
stuff is on my web site).

To bring this back to Leicas; my 21/3.4SA has essentially no distortion,
and my 21/2.8ASPH has a very slight bit, and my Nikon 20/2.8 has a little
bit more, but the perspective rendering is essentially the same; the minute
difference being caused by the slightly higher linear distortion levels of
the two retrofocus designs.


   *            Henning J. Wulff
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