[rollei_list] Re: Really Off-Topic, paraxial lenses for digital cameras
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Wed, 1 Jul 2009 15:59:04 -0700
----- Original Message -----
From: "Don Williams" <dwilli10@xxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, July 01, 2009 3:24 PM
Subject: [rollei_list] Really Off-Topic, paraxial lenses for
digital cameras
I know this is really off topic but I also know there are
several
folks in this group that can quickly answer my questions.
(I also note that there is a gradual acceptance, even
among the
die-hards out there, that digital photography has at least
some place
in the world today. To those I apologize for even asking
these
questions, and for the loss of MF Kodachrome.)
There is a common term used by various vendors, "digital
lens". It's
obvious that there is nothing "digital" about the lens.
What the
term "digital" refers to is how the light is delivered to
the sensor.
My thought, based on attending several trade shows in
Vegas in the
days of trade shows, is that a digital lens is one that
presents all
the light for a specific point on the sensor at a 90
degree angle to
the sensor. Olympus makes much of this in their
discussion of
paraxial lenses and their four-thirds system.
The sensing elements in a digital sensor are separated by
ridges, the
height of which I don't know. Any light destined for a
specific
sensor will be shadowed by the rims of those ridges-
Imagaine the
effect you would have with a standard photo lens if the
ridges were
1/4" deep. Only the elements in the central zone of the
sensor would
get any light. The sensors at the edge would be
completely shadowed
by the rims of the ridges. Of course that's an
exaggeration, but as
a mental exercise it is useful.
My guess is that a regular lens designed for film work
would be just
fine if the distance from the real element to the sensor
is large,
but if that distance is short, the film-designed lens
would have
brightness fall-off at the edges.
I have done some searches recently on "paraxial" lenses
and get so
many hits that I can't find the ones I used to see. Also,
Olympus
USA still uses a description for their Zukor lenses that
mentions the
90 degree incidence angle, but all technical content
beyond that has
been dropped.
Richard, Emmanuel, and also those who have actually
designed digital
systems and scanners, and anyone else out there, can you
help me on this?
Richard- just one page please, something I can understand.
(I still don't understand why my film scanner doesn't care
very much
if the film is not flat against the glass, how does it
still produce
sharp scans?)
Finally, two more questions-
1. What has happened to the Bayer sensor design?
2. Is it true that Kodak is the biggest sensor producer,
in terms of
quantity, or is that just a rumor?
DAW
You mean telecentric. Telecentric lenses have one pupil
at infinity. While it seems counter intuitive that means the
light at one side of the lens is parallel. Such lenses have
been used for many decades in profile projectors because
slight movement of the object does not affect the size of
the projected image. In the case of a lens made for a
digital sensor where the actual sensing points are in a
shallow tube using a telecentric lens with the exit pupil at
infinity prevents light being blocked by the tube. I don't
have a simple explanation of how such lenses are designed
but they are common and the design technique is well known
and covered by standard texts on lens design. No magic, just
more expensive.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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