Vertical error and leveled camera MISCONCEPTION!

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

BobH comments on the vertical misalignment error found in certain cameras
and writes:

>I think this is not a fixed error but is the result of not holding 
>the camera LEVEL...

Bob, with all due respect, I have to say that this is not true!  It is a
common misconception that you will get an alignment error if you tilt the
camera.  The fact is that there is nothing you can do in terms of holding 
a camera that will give you a vertical or any alignment error.  Even if you
tilt the camera 90 degrees (by holding it vertically) you will still get a
perfectly aligned pair!  "Aligned pair" means that the line that connects
the same object (point) in each frame is parallel to the edge of the film
(and later parallel to the line of the eyes).  This happens if the lenses
are parallel and the line connecting their centers is parallel to the edge
of the film.  It is a matter of having the lenses aligned with respect to
the film (or the film aligned w/respect to the lenses), a matter of design
that has nothing to do with the way the camera was held when the shot was
taken.

What will happen if you tilt the camera sideways?  The actual physical
horizon will not be leveled but will appear tilted.  If there is a clear
pronounced horizon in the picture then this is bothersome.  You can try and
correct this by rotating both frames so that the horizon becomes leveled
again but by doing this you introduce real misalalingments (the kind that
will give you a headache in viewing).

Note that if there is no horizon or sense of any orientation in the picture
(it is difficult to think of an actual situation where this is the case,
but it can be done, one typical example is when you shoot from the top
looking directly down) then shooting with the camera leveled means
nothing because "level" is not defined.

Let me repeat this because many people have it wrong:  If one of the two
images is in a different height (seen when mounted in a "squared" mount,
note: defective mounts have been reported from time to time) then the
problem is with the camera and it is a matter of lens' or lens to film
alignment.

Holding the camera tilted with respect to the horizon ("out of level") will
not result in an alignment error.  It will just result in a tilted ("out of
level") horizon.

George Themelis


------------------------------