When the projection distance (f1) matches the focal length the beams of light will leave the screen in parallel lines. In this case we say that the second conjugate (f2) is infinity (∞).
By designing the screen’s focal length to match the projection distances used in practice it is possible to ensure an optimum light dispersion from the screen.
When selecting the screen/projector/lens combination, the projection distance should match the screen focal length to within the stated tolerance.
If the projection distance is shorter than the focal length, the light dispersion will be widened.
This means the closer the projector gets to the screen, the corners of the image become less bright.
If, on the other hand, the projection distance is longer than the focal length, light dispersion will be narrowed. The further the projector is from the screen, the brighter the image will be in the corners. This is useful for maximising screen performance for close viewers.
By careful design, the screen’s focal length can be used to optimize the light dispersion from the screen in specific applications.