In article <59p3ie$asb@belize.it.earthlink.net>, johnliu@earthlink.net says...

>Can anyone tell me something about tilt and shift lenses? I like to
>photograph buildings and will be taking an extended European trip where I
>plan to visit a lot of cathedrals. Will a TS lens really
>allow me to photograph a cathedral without the "leaning back" effect?
>Do they really work? How much can the lens correct
>(that is, I assume you can't stand at the foot of the World Trade Center
>and produce a photo that looks like you are a mile away.) Is the TS lens
>for the Canon FD a good one?

As to the last question, there is only the 35mm Canon TS - probably good,
but I am a "Nikonophile", so unfamiliar with it.... (though, as I recall
from handling one, the aperture is only manual (not even preset), and the
tilt and shift functions are on axes 90 degrees apart, so they can rarely
be used simultaneously (too bad!). 35mm is also not wide enough to be
very interesting much of the time for perspective control use.
As to the first, they can be used to "straighten" buildings, within limits.
According to the rules of rectangular perspective, if the horizon line is
placed dead center in the image area of an unshifted lens (frame coverage
centered in the image circle [with no tilt]), by definition all the lines
perpendicular to the horizon line are parallel in the image (though most
wide-angle lenses for 35mm cameras have some linear distortion, so the
lines will probably not be perfectly parallel...). If you imagine the
coverage of the lens being an image circle larger than the 35mm frame,
and the shift mechanism as a way to move the 35mm frame around inside the
circle to crop out of the circle just what you want, you have the idea
of how PC (shift) lenses work - you level the camera (which places the
horizon line in the center), and shift the lens (frame) until you have
covered in the frame what you want. BTW, if you try moving a rectangular
frame around in a circle a bit bigger than the rectangle, you will see
that you can get more shift with a horizontal frame than with a vertical.
Most PC lenses allow 11mm of shift with a horizontal frame, allowing the
horizon line to be placed 1mm above the bottom edge of the 35mm frame.
Unfortunately, for sharp coverage with vertical frames, 8mm is the limit
(though Nikkor PC's will illuminate the full 11mm shift, and sharpness can
be good to the corners with small apertures), which places the horizon line
well above the bottom of the frame, alas.
As for tilt, the effect is to change the angle of the plane of sharpest
focus. Normally, the lens is mounted parallel with the film plane, and
the lens focuses a plane (one hopes! ;-) parallel with the film. If you
tilt the lens, the line of intersection of the extended plane of the
film and the plane running through the optical center of the lens is
also the line of intersection of the subject focus plane. In practical
terms, if you aim an untilted lens at a flat alfalfa field and take a
photo at a very wide aperture, you will get a photo showing a line of
sharp alfalfa where the focus and subject planes intersect (everything
else will be less sharp). If you observe where the film plane (extended
to meet the ground plane) intersects the ground plane, and tilt the lens
so that the lens plane has the same intersection line, the ground plane
will be rendered sharp all over on the film (but anything extending out
of the ground plane [like trees, buildings, etc.] will be out of focus).
Usually, a compromise plane which sort of covers the various parts of
the subject, combined with a smaller aperture, is needed for full DOF
coverage - but adding tilt to using a small aperture makes it easier
to get sharp images with great DOF with a deep subject. In 35mm, there
are a few subjects which benefit from tilt, but it is less important
than it is with larger formats where DOF is more limited with a given
aperture and angle of view - most of the time, f16 will provide
sufficient DOF with 35mm FL lenses (and shorter) with 35mm format.
Hope This Helps