Howard,
Besides the diagrams there is a statement on that page that is also misleading and it has caused much grief (to some people) and it is:
"... the smaller sensor size engendered a change in the focal length of lenses. Typically, a lens designed for 35mm film would gain by a factor of 1.5X, a change that could have a dramatic impact for photographs that necessitate a wide angle."
Sorry. The focal length of a lens does not change by being attached to a camera that has a smaller or larger format image area.
Now as for the lens diagrams ... they are extermely simplified to the point that they would lead one to believe that light rays leave the rear element of the D lenses in parallel fashion. If light did indeed leave the lens in such a way an image would not be formed. Or, the area of the subject that such a lens would cover would be no bigger than the circle of illumination of the lens such as in a confocal optical system.
OK ... now if the exit pupil of the lens were to appear to move up, down, or
sideways as one looks at the rear of the lens from the image plane it is
possible that the cone of light forming any image point will arrive at the
image plane more normal to the surface than if this is not the case. This would
be highly desirable and probably (I am not sure about this) is what is being
done.
In the illustrations provided there is no reference to a cone of illumination for a given image spot. Further, the light rays for a conventional lens are reduced to a single light ray something that never happens in reality. There are always two cones of rays at work. One emanating from every point in the subject and filling the "entrance pupil" (simplify to diaphragm) and the other focused at the corresponing location in the image plane emanating from the "exit pupil" (simplify to diaphragm) of the lens.
If you could make the lens in such a fashion that the cone of illlumination
reaching any given image plane spot would be normal to the surface - well,
you'd have the "digital lens". I am not disputing that this can be done. In
fact I am (almost) sure it can be done. It is just that the illustrations are
oversimplified and they give the "wrong impression". They are also designed to
overemphasize the defects that might become evident if a non "digital" lens is
used on a camera equipped with a CCD sensor. Why? PR imho.
Alternatively an optical design might be fabricated where the lens sends more
light to the edges than the center (a kludgy alternative would be a center spot
filter located behind the lens). This would correct for fall-off but other effects could remain.
Anyway, I am no optical designer nor an optics teacher or a manufacturer. My
statements are simply personal opinion and may or may not be totally accurate
or correct. I think I am not totally wrong but I will admit I could be. It is
just that advertising hype bugs me when not clearly supported by facts. If I
have offended anyone for expressing my opinion I apologize. I guess I could
keep quiet.
well, arrivederci for now.
andy
let's assume that this was not intended and that through some artistic
license one made a beam of uniform width leave the lens while at the same time
still bringing rays emanating from subject points to a focus
But assuming that one could reduce the
aperture of a lens so that only a single light ray were associated with a
single point in the subject
ADavidhazy wrote:
I think that the drawings supplied or based on illustrations provided by Olympus are misleading.
Further, basically incorrect in my opinion.
Andrew Davidhazy, School of Photographic Arts and Sciences/RIT
andpph@xxxxxxx http://www.rit.edu/~andpph
Why? Unless that's too complicated for the forum (and me in particular!)
It seems surprising that Olympus would deliberately supply misleading diagrams - or else they don't understand their own technology!
Howard
