Projection throw distance – the distance that the projector must be from the screen in order to fill that screen – is mainly a concern if using rear screen as it takes away from seating space in the meeting room. Most LCD projectors have a rear screen and front screen setting. Whether or not you are using front of rear, projection distances are the same.

Lenses and throw distances for LCD type projectors are given in ratios to screen width. For instance a 1.5 :1(1.5 to 1 ratio) lens will need to be 1.5 times the screen width in order to fill the screen. So if you have a 10 ft. screen, with a 1.5:1 ratio lens, the projector will need to be 15ft. from the screen. A lens with ratio of 2:1 (2 to 1 ratio) will need 20ft. to fill the same screen.

Large Format Projectors:
Most large format LCD, DLP, Light Valve projectors have interchangeable lenses. So you can specify certain lens ratios. Most common for short projection distances are 1.2 to 1, and 1.5 to 1. Most large format LCD projectors have these for closer projection throws. There are also many longer throw lenses, and zoom lenses. Check with your AV equipment supplier to make sure of what they have or can get. Remember a smaller ratio lens 1.5:1 is a shorter projection throw distance than a 3:1 ratio lens.

Small Format Projectors:
Most small projectors do not have interchangeable lenses. Each manufacturer uses its own standard zoom lens. Most standard is probably a zoom lens that will go from a 1.9 to 1 ratio to a 2.5 to 1 ratio.

With smaller projectors, it is safest to check on the lens ratio by each model, but for a rule of thumb, use 2:1 as the ratio you for a quick calculation, then consult your equipment supplier.

Screen Ratios and Throw Distances:
	Lens Ratios
	1.5 to 1	2 to 1	2.5 to 1
8 ft screen	12 ft.	16 ft.	20 ft.
10 ft screen	15 ft.	20 ft.	25 ft.
12 ft screen	18 ft.	24 ft.	30 ft.
14 ft screen	21 ft.	28 ft.	35 ft.
16 ft screen	24 ft.	32 ft.	40 ft.
20 ft screen	25 ft.	40 ft.	50 ft.