At 06:26 1/3/02, Joel Wilcox wrote:
At 09:35 PM 1/2/2002 -0800, you wrote:
Some reason I like this picture quite a bit:
http://www.dragonsgate.net/richard/gaslamp.jpg Comments? I am a bit
surprised by the building distortion on the left, as I'm pretty sure I
was using the 50mm lens at that time.
This is actually a comfort to me. I have a photo I like a lot of some
local buildings and despite using a 50mm lens for the expressed purpose of
avoiding the keystoning effect of the wides, the lines of one of the
buildings toward the edge of the FOV has a similar goofy distortion.
John Shaw (I think) encourages one to think of the 50mm as a "long
WA". Perhaps that's more than a mental game. One of the technical list
members will surely have an explanation. Is this a "non-flat-field" type
of problem?
Joel W.
I don't think this is a "flat field" issue. That relates more to critical
focus being a flat plane in space parallel to the film plane.
This image has "keystoning" because the rectilinear lens did exactly what
it was supposed to do. The lens was tilted upward from dead
level. Vertical lines in the street lamp and buildings were no longer
parallel to the film plane. As soon as the camera was tilted upward,
vertical lines were no longer parallel to the film plane. The result is
converging vertical perspective lines. It looks much more natural if they
are horizontal lines; e.g. photographing the corner of a building with roof
lines converging to the ground as they recede toward the
background. Exception: a very radical tilt upward which gives the viewer
an expectation the vertical lines should converge. Avoiding converging
vertical lines is one of the reasons the shift lenses were created.
With a rectilinear lens, the *only* parallel lines in space that will be
parallel on the film plane are the ones parallel to the film plane.
-- John
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