Throw Ratio Explained

The throw ratio is a way of expressing the relationship between 1) the distance of your projector to the screen and 2) the size of the projected image. It is used as away to describe the angle of the light path as it leaves the projector and continues on to the screen. The reason why this is important is that this lens has a limitation where if the throw ratio is too small (less than 1.75) the lens will cut off the sides of the projected image and cause what is known as vignetting.

The following is a series of examples that show how the calculation of a throw ratio can be determined. Also, note that you can use the Throw Ratio Calculator, located on the right of this site, to determine throw ratios.

Here we go…

Example 1:
Let’s say you have a projector positioned 15ft from the screen. Your desired projected image is a width of 7.5ft.

The resulting throw ratio would equal 2.0 (15ft / 7.5ft)

Example 2:
Now let’s say you use the projectors zoom function to create a projected image that is 10ft in width…remember your projector is still 15ft from the screen.

The resulting throw ratio would equal 1.5 (15ft / 10ft)

Example 3:
Finally, let’s say you are considering the purchase of a JTX Dual Prism Anamorphic Lens. From your research on this site you learn that it is not compatable with throw ratios less than 1.75. You want a projected image of 10ft in width and your projector is 15ft from the screen. We have already determined in Example 2 that the throw ratio would be 1.5. Since the lens requires the throw ratio to be greater than 1.75, the lens will not work in this theater setup.

However, you do have some options:

1) If your room allows, you could move the projector back further against the rear wall. Let’s say you move it back an additional 3ft for a total distance to screen of 18ft. Again you want a 10ft projected image width.

The resulting throw ratio would equal 1.8 (18ft / 10ft)…this works

2) You could decide to leave the projector in place at the distance of 15ft to screen, and instead go with a slightly less projected image width. Let’s say instead of 10ft, you modify your setup to be 8ft in width.

The resulting throw ratio would equal 1.875 (15ft / 8ft)…this also will work

It should be understood that the throw ratio limitation (of 1.75) of the JTX Dual Prism Anamorphic Lens may require modifications to your existing theater setup. Only you can determine if these changes make sense to implement, in order to integrate this entry level lens.