In article <4ne6or$ls4@spam.maths.adelaide.edu.au>, dcook@maths.adelaide.edu.au says... [..]

>In particular, I'm curious about tilt/shift lenses. I understand how
>shifting a lens can correct converging vertical lines in a photo >(having performed a simple experiment with my camera and an unmounted >lens :). How much correction can you gain this way - e.g., if standing >across a road from a tall building, how much of the building can be >included in the picture ?

It depends on too many things to tell. PC's from different mfgrs. can
be 24mm, 28mm, 35mm, 45mm, or 90mm. Some will shift up to 11mm from
center, which means that with the camera leveled, the horizon line
will be 1mm from the bottom of a horizontal frame, and 7mm from the
bottom of a vertical frame (which requires the most stopping down to
cover the top of the photograph sharply, and the top corners may be vignetted if a thick filter and/or shade is used).

>Also, what does tilting a lens accomplish, and how ?

Tilting of a lens allows correct focus on a subject plane that is not
parallel with the film plane. When the lens is tilted, the (extended) planes of the film, lens, and subject intersect in a line. This allows
wide-aperture photos of things like fields, water surfaces, table-tops, etc. to be sharp all over (though things sticking out of the plane
(like trees, rocks, buildings, boats, etc.) will not be sharp (which requires a compromise tilt, with stopping down for greater DOF. The
technique permits greater than normal DOF with many photos than would
be possible with use of small apertures alone.

>Finally, is it possible to get tilt/shift lenses in either screw mount >or Pentax K-mount, and what sort of price would I be looking at ?

Pentax made a 28mm PC (no tilt, but for 24-28mm lenses, it isn't a
great loss most of the time), as did Schneider (35mm f4, with
different mounts available, as I recall). There were a couple of tilt/shift bellows made that would work with lenses of about
100-150mm for infinity work.
Hope This Helps