Quote Originally Posted by Anon Ymous View Post
Or use a ruler to check if the parallel sides of the
rectangle are equal.
First check with a square to confirm that it is
a rectangle. It could turn out to be a
parallelogram.

BTW a projected image may be sharp but not
square. For the projected shape to be true to
the object being projected the optical axis
must be perpendicular to the planes of
both film and baseboard. That is not
the case for sharp. Dan