- Distance = distance from projector lens to screen
- Width = width of projection screen
- Ratio = lens ratio, e.g. 1.3-1.7:1 or 2.7-3.5:1 (the range is the zoom range if the lens is a zoom lens)
Distance / Width = Ratio
Examples:
A 11x20' projector screen is 30' away from projector. What lens is needed? 30/20 = 1.5 So a lens whose outer bounds are above & below 1.5 works best, such as a 1.3-1.7:1
A 24' wide screen is paired with a 1.3.-1.7:1 lens. What distances are in range for the projector? Closest projector can be is 31.2' x/24 = 1.3 x = 1.3 * 24 x = 31.2' Farthest away projector can be is 40.8' x/24 = 1.7 x = 1.7 * 24 x = 40.8'
With this screen & lens, the projector should be somewhere between 31.2 - 40.8' away from the screen, ideally in the middle of that range for maximum flexibility.
What is a throw ratio for a projector?
For any given projector, the width of the image (W) relative to the throw distance (D) is know as the throw ratio D/W or distance over width. So for example, the most common projector throw ratio is 2.0. This means that for each foot of image width, the projector needs to be 2 feet away or D/W = 2/1 = 2.0. So if I'm using a projector with a throw ratio of 2.0 and I have an image width of 5 feet, then my throw distance must be 10 feet. So the throw ratio is a simple formula that let's you easily compute throw distance or image width given that you know one of these measurements. A projector zoom lens will have two different throw ratios, one for the minimum zoom setting and one for the maximum zoom setting.