wwp@yahoogroups.com:
Re: Twenty Questions for the VR Photographer
Erik Krause 2005-Sep-16 21:18:00
Am Friday, September 16, 2005 um 17:17 schrieb Scott Highton:
> A retrofocus design shifts the nodal planes of the lens, providing a
> short focal length with a long back focus distance. This allows for
> use of the lens with reflex viewing -- a necessity for composition,
> focusing, etc. Thus, the "rear" nodal point winds up positioned near
> the front of the lens, and the "front" nodal point winds up near the
> rear of the lens. That's the simple version, I think (grin).
Retrofocus design is as old as the single lens reflex camera and does
not affect the basics of optics. And nodal point is a quite confusing
thing, since it is often mixed with entrance pupil. Optics knows a
thing called 'principal plane' (in diagrams usually called H), which
only an ideal thin lens has one of. Any real lens has two. Our 'no
parallax point' is where the optical axis meets the front principal
plane (H1).
The principal planes in modern lenses don't have much to do with the
physical position of the actual lenses - they can be outside the
lens. The rear principal plane of a usual telephoto lens is in front
of the front principal plane and often in front of the front lens -
this is because the rear principal plane is defined as the point from
where focal length is measured. See here:
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/verpow.html
A 20mm lens stays a 20mm lens, whether it is retrofocus or not, and
for that lens the gaussian lens equation applies: 1/f=1/i+1/o. i is
the image distance and measured from the rear principal plane to the
image plane, o is the object distance and measured from the front
principal plane to the object:
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/imgpri.html
As you can see from the diagrams at
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenseq.html
a light ray that goes through the middle of a thin lens (where the
optical axis meets the principal plane) emerges in the same
direction. For a thick lens a light ray that goes through the middle
of the front principal plane emerges from the middle of the rear
principal plane. Thats the cause why the middle of the front
principal plane is our 'no parallax point'.
best regards
--
Erik Krause
Offenburger Str. 33
79108 Freiburg