How far from the screen do 10x10 transparency projectors
need to be positioned ?

10x10 transparency projectors, (commonly called "overhead" projectors)need varying distances from the screen, depending on how far the
furthest person is from the screen.
The chart below shows the approximate relationship between distance to the furthest viewer, and the distance that the transparency projector needs to be from the screen.

Given a certain focal length lens, there is a formula that is sometimes used to determine the approximate distance between the projector and the screen, but it has drawbacks. For one thing, it often doesn't make it clear whether the measurement is to the 1)front of the projector, 2) front of the lens, or 3) back of the projector. In addition, the projector has to be in some kind of stand which can be large or small and must be accounted for in laying out legal-sized cross-aisles.

Solution? Don't trust formulas (or even the chart below)!

Without setting up the intended projector in the intended stand and focussing it on a screen installed in the intended place, you can't be sure that the resulting student cross-aisle will meet fire codes.

Chart above is approximate from the screen to the student-side of the stand...
for projectors with a 355mm lens, and installed in a moderate-sized stand.

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