Focal length/ Focal Distance

It is the distance from the surface of the lens to the focal point or other words it is the distance between the center of a lens and the spot where the picture is beginning to focus.

The focal length of a lens determines how wide an angle the image is projected. In simple terms the larger the number of the focal length of the lens the narrower the beam. The lower the number of lens focal length the wider the angle. This lens beam angle is called the 'inclusive' angle.

Color video image projection apparatus as claimed in claim 1, wherein said projection lens has a focal length between 0.5 and 0.9 times the diagonal of the image raster on the faceplate of each of said display devices.

Projector Screen Size and Brightness Calculators

http://www.diyprojectorcompany.com/theory/calculator.htm

http://avandmore.net/ProjectorScreenCalculator/tabid/63/Default.aspx