----- Original Message ----- From: "Eric Goldstein" <egoldste@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Sunday, March 11, 2007 7:38 PM
Subject: [rollei_list] Re: rollei, level
This is often referred to as projection distortion by map makers and
others who must convert 3 dimensional space into 2d...
Eric Goldstein
--
My stuff snipped....
When a two dementional surface is photographed using a standard
lens the result will be an exact duplicate. Suppose you photograph a
section of graph paper, the resulting negative (and print) will have
exactly square divisions of exactly the same size all over, assuming
for the moment a perfect lens. If one considers the viewing conditions
in life a wall with equal size squares on it will be affected by
diminishing perspecitive so that the squares at a distance will appear
smaller than those on the viewing axis. This effect is reproduced in a
photo by having the squares of equal size provided the viewing
distance is correct. If one considers three dimensional objects the
results are somewhat different. If one photographs an array of golf
balls those at the corners will appear larger on the print than the
one at the center and will also be egg shaped with the narrow end
facing the center. When viewed from the center and at the right
distance, the diminishing perspective as it applies to the print will
make the balls again appear round and of the same size. I think the
term for this is orthographic. Arial mapping requires a different rule
for angle of view vs: angle of projection. In some cases, such as the
famous Zeiss Pleon lens, a corrective projection lens is used to
achieve orthographic projection for mapping. This corrects for the
diminishing perspective of objects far from the camera so that the
scale of reproduction is constant throughout the area of the image.
This would also be the ideal condition for a panorama made up of a
series of pictures, each with a different vanishing point. A panoram
made using a rotating camera does not have a definite vanishing point.
Rather, it has a vanishing line, usually at the vertical center of the
picture.
Another comparison is between a panoram made using an extremely
wide angle lens, cropped to a narrow vertical angle vs: a rotating
camera. The perspective presented by the wide angle photo is such that
it looks right when presented as a flat surface, i.e., the diminishing
perspective makes the picturer look right when viewed from a definite
central point, namely the projection of the vanishing point. The
picture made by the rotating camera look right when presented as a
cylindrical surface with the viewer in the center.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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