[rollei_list] Re: OT Schneider 21mm f/3.4 Super Angulon for Leica M
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Wed, 24 Feb 2010 18:57:23 -0800
----- Original Message -----
From: "Eric Goldstein" <egoldste@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, February 24, 2010 1:57 PM
Subject: [rollei_list] Re: OT Schneider 21mm f/3.4 Super
Angulon for Leica M
It certainly can be... my point is that they are two
different effects with
two different causes. Do we know which is at work on this
particular lens?
Is it a combination of both?
Eric Goldstein
Mechanical vignetting is the obscuring of part of the
aperture by the lens mount. You can see this effect by
looking back at the lens from the corners of the film gate.
Its easier to see in a view camera than in a smaller camera
like a rollei but its there. After you stop down
sufficiently the entire aperture will be visible.
Part of the fall off is from the "distortion" of the
aperture. If you look at the iris staight on from the center
it is round shaped but as you move toward the periphery of
the image the iris becomes cat's eye shaped. So its
effective aperture becomes smaller. Since it is larger in
one direction than the other the resolution limit due to
diffraction will vary with whether you are seeing light
coming through the long way (higher resolution) or the short
way (lower resolution). Another factor is the inverse square
law between the lens and film. At the center of the image
the distance is least. It becomes larger as you move toward
the periphery so there is some fall off due to this law.
Because the angle of the cone of exiting light is smaller
for a retrofocus lens the fall off due to this effect is
less.
Roosinov found that if he introduced some coma into the
stop its shape would then be magnified for larger image
angles. The amount of coma can vary in different designs but
accounts for as much as one term of the cos^4 theta factor.
Most photographic lenses are orthogonal. That means
that if you view a cross-hatched surface like graph paper,
it will be reproduced with the cross-hatches all the same
size and with the stright lines staight. Since the lens is
seeing the periphery of the _object_ at a greater distance
than the center a true reproduction would be to see the
outside squares diminish in size. Actually, the diminishing
perspective seen by the eye is duplicated in the image
provided that its viewed from the right distance. Then the
outside squares will be seen as being smaller even though at
a longer distance they will all be seen to be the same. This
also results in a single term of diminishing light.
This same effect shows up on three dimentional objects
in a different way. When one photographs an array of golf
balls only the one in the center will be reproduce round.
Those toward the periphery will become increasingly
egg-shaped. When seen by the eye at the correct distance the
diminishing perspective will make them look right again. The
reason for "distortion" in orthogonal wide angle lenses is
from viewing at too great a distance for the eye to see the
same perspective the lens does.
Lenses which are not orthogonal, such as a fish eye
lens also do not have as much fall off. They give a similar
effect to viewing an image reflected by the surface of a
sphere. These images can not be corrected by changing
viewing distance but can be made orthogonal by a corrective
printing lens. Such combinations were used in aerial
mapping, the Zeiss Pleon being an example.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
Rollei List
- Post to rollei_list@xxxxxxxxxxxxx
- Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe'
in the subject field OR by logging into www.freelists.org
- Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with
'unsubscribe' in the subject field OR by logging into www.freelists.org
- Online, searchable archives are available at
//www.freelists.org/archives/rollei_list
Other related posts:
