and the throw ratio is given as a range (e.g. 1.2 - 3.0). This provides flexibility in positioning the projector, since the lens can be adjusted to accommodate any throw ratio within that range. EXAMPLE: A video projector with a throw ratio of 1.2 - 3.0 is to be used on a screen that is 100" wide. Starting with the smaller value, we use the formula above (D = W x R, or D = 100 x 1.2) to determine that the closest position possible is 120". The farthest position for this projector is then 300". Installing this projector anywhere within the range of 120" to 300" from the screen will allow us (by adjusting the zoom setting on the lens) to fill the screen. CAUTION: In some projector/lens specifications, the ability of a lens to adjust (zoom) its throw ratio is given as yet another ratio. This can be a source of confusion. For the above example, a manufacturer might say that the lens has a zoom ratio of 2.5 (3.0 / 1.2 = 2.5), or a "2.5:1 zoom lens". A greater zoom ratio means that the lens is able to adjust more, but it says nothing about the specific throw ratios the lens is designed for. Another lens with a throw ratio range of 2.4 - 6.0 would have the exact same zoom ratio as the previous example. The "zoom ratio" is more useful for marketing purposes, while the actual "throw ratios" are necessary for designing a projection system. In some cases, the term "zoom ratio" is not used, which can contribute to confusion. If a single value is given (not a range of values) and the lens is capable of zooming, the value is likely the zoom ratio. A relatively small percentage of lenses have no zoom capability (fixed lenses), so their actual throw ratio would be given as a single value. References External links Projectors