Calculating Throw Ratios

 

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Each projector manufacturer has a choice of which type of lens to place on their projectors and, understandably, each lens has a different set of characteristics. One of the more important characteristics of a lens is its throw ratio. Throw ratio is the distance (D) from the screen that a projector needs to be in order to create a specified size image.

Throw ratio is usually defined by the width of the image. Some manufacturers give the throw ratio as diagonal, however this can lead to confusion because the diagonal of an image changes depending on the aspect ratio of any given projector.

To keep things simple, we'll stick to the equation of Distance/Width (W) because the width of an image never changes. Now, each projector's lens has a set D/W ratio. If a projector has a D/W ratio set at 2, then it needs to be two feet away from the screen for every one-foot of image width.

To further explain this, let's look at a real world example. After determining the available space in his room, John would like a five foot-wide image. So based on a D/W ratio of 2, let's figure out the distance that John's projector needs to be to display a five foot-wide image. We'll use this equation: D/5=2 or 5 x 2 =10. According to the calculation, John will need to place the front of his projector lens 10 feet back from the screen in order to achieve his goal of a five foot-wide image. 

What about zoom lenses?

 This would always be a simple calculation if projectors were not equipped with zoom lenses, and all Optoma projectors (except EzPro 731) have zoom lenses. Most of us are familiar with the zoom lenses on camcorders and cameras. A zoom lens on a projector has the same purpose: it allows the projector to vary the image size from a minimum to a maximum.

If we think back to the throw ratio, this can actually be turned into a simple formula. We know that a projector's zoom lens can achieve a throw ratio from any minimum to any maximum. We'll use 2.0-2.5:1 for this exercise. This simplifies figuring out how far back your projector needs to be to accommodate your available space. Take the distance you want to move your projector back (D) and divide it by the width of the image you want to project (W). If the resulting number falls in between a projector's stated throw ratios, then the projector will perform what you are asking of it. Let's take a look at specific scenarios (below):

1.  I know the width of the screen, but I'd like to know how far back I should place my projector.

To answer this question, we'll refer to a projector with a throw ratio of 2-2.5. For a screen width of five feet, the projector may be placed anywhere between 10 feet to 12.5 feet back from the screen. To determine this, we used the following calculations: Width (W) x Throw ratio = Distance (D). Using the throw ratio range of 2-2.5 we calculated: 5 x 2 = 10 and 5 x 2.5 = 12.5.

2. The projector needs to be 13 feet back from lens to screen. How
large of an image can I get?

Again using a throw ratio of 2-2.5, 13/2 = 6.5 and 13/2.5 = 5.2. This means you can create a screen that is as small as 5.2 feet to as large as 6.5 feet. To calculate this, we used the following formula: Distance (D) / Throw ratio = Width (W).

A common misconception is that a projector's zoom lens can create very large minimum and maximum throw ratios. It is important to remember that all factory zoom lenses will only allow for small variations in throw ratios.