Keystone Effect and The Solution

The Keystone effect, also known as the Tombstone effect, is caused by attempting to project an image onto a surface at an angle, as with a projector not quite centered onto the screen it is projecting on. It is a distortion of the image dimensions, making it look like a trapezoid, the shape of an architectural keystone; hence the name of the feature. In the typical case of a projector sitting on a table, and looking upwards to the screen, the image is larger at the top than on the bottom. Some areas of the screen may not be focused correctly as the projector lens is focused at the average distance only.
Solving the problem

The problem arises for screen projectors that don’t have the depth of focus necessary to keep all lines (from top to bottom) focused at the same time. The problem can be solved by:

Move the projector more to the center of the screen, if this does not interrupt vision
Tilt the screen in a small angle
Use some type of software on the projector (or computer controlling the projector).