wwp@yahoogroups.com:
Re: Twenty Questions for the VR Photographer
Scott Highton 2005-Sep-16 17:17:00
--- In #removed#, "Erik Krause" <#removed#> wrote:
> They are simply wrong (a typo I suppose) as you can see if you think
> about the above statements: The rear nodal points distance from the
> film or sensor is exactly the focal length. This would be far inside
> the camera for a 8mm fisheye. The real "no parallax" point is very
> near to the front lens...
Hi Erik,
Modern lens design is far more complicated than most of us care to
even know about (myself included). A degree in physics or optical
engineering is probably helpful for those that do.
However, the Focal Encyclopedia description of nodal points/planes is
accurate, as far as I can tell (it's authors ARE such technical
experts from Rochester Institute of Technology - RIT).
For traditional lens design, your statement above about focal length
is true. This is illustrated by the characteristics of early fisheye
lenses, such as the 8mm and 10mm Fisheye-Nikkors (f/8 and f/5.6
respectively) from the 1960s, where the rear element(s) of the lens
protruded so far back into the camera body that the lens could not
even be mounted without the camera's mirror being locked up out of
position.
To overcome this problem, Nikon switched to a retrofocus design for
their later (1970s) models, particularly the Fisheye-Nikkor 8mm f/2.8
and the monster 6mm f/2.8.
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).
If you really want some heavy technical reading (guaranteed to put you
to sleep at night) in addition to the Focal Encyclopedia references,
take a look at Sidney Ray's Applied Photographic Optics book (also
available from Focal press, or via Amazon).
Other online sites that might be helpful include:
http://www.naturfotograf.com/lens_fish.html
http://www.digitalbooks.de/nikon/comp9.htm
Regards,
Scott Highton
Author, Virtual Reality Photography
http://www.vrphotography.com