In video projection terminology, throw is the distance between a video projector lens and the screen on which it shines. It is given as a ratio (called throw ratio), which describes the relationship between the distance to the screen and the width of the screen (assuming the image is to fill the screen fully). Throw ratio is a characteristic of the lens of the projector (although "projector throw" and "lens throw" are often used synonymously). Some projectors (typically larger, more expensive ones) are able to accept a variety of lenses, while lower cost projectors tend to have a permanent lens that is not designed to be changed. Some lenses are fixed at a specific throw ratio, while most are adjustable (zoomable) to a range of throw ratios. Distance to the screen is measured from the front of the lens. Formula Distance (D), Width (W), Throw Ratio (R) If the distance and width are known, calculate the throw ratio using the formula: R = D / W If the screen width and throw ratio are known, calculate the distance using the equivalent formula: D = W x R Although it is often stated as a single value (or range of values), throw ratio is a comparison of D : W. To reduce this to a single number (as is typically seen in projector/lens specifications), start by dividing both sides by W, leaving us with D / W : 1. The 1 on the right means "for each one unit of width of the screen, how many units away should the projector be?" In practice the 1 is often assumed (omitted) when listing this specification. Examples (fixed lenses) A video projector (lens) with a throw ratio of 2.0 (or "2.0 : 1") would need to be positioned at a distance that is twice the width of the screen. So if the screen is 60" wide, the projector needs to be 120" from the screen. A video projector (lens) with a throw ratio of 0.4 or less would be positioned relatively close to the screen, and would be considered a "short throw projector". A video projector that must be positioned very far from the screen would need a "long throw lens" with a throw ratio like 7.5. Zoom lenses While some lenses have a fixed throw ratio (as in the examples above), most are adjustable. This is referred to as a zoom lens,