Viewing angle: In projector technology and with rear projectors, the viewing angle indicates the distance from the center line at which the image can still be seen well and without restrictions. Without further information, the horizontal viewing angle is usually meant, which is of less importance in practice, but in principle the vertical viewing angle is just as important. A good horizontal viewing angle is around 110 to 130 degrees (ie 55 to 65 degrees to the left and right of the center line).
Image quality may progressively and disproportionately decrease as the distance from the centerline increases. Cause can at the front projection a canvas with too high Gain factor be (which incidentally also has a disturbing hot spot can generate). In the case of rear projectors, the influence of ambient or scattered light plays a role. This is why it can happen, especially with rear projectors, that only a dark pane can be seen even with small deviations from the optimal viewer position.