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What is Projector Lens Throw?

The lens throw describes the size relationship between the projector and the projected image. There are three common ways of describing the lens throw

Throw Distance - the distance between the projector lens and the projected image

Throw Ratio - the ratio of the throw distance to the projected image width

Throw Range - the minimum and maximum Throw Ratios of the projector lens

Throw_Distance

Why is Projector Lens Throw Important?

In most cases, the goal is to project an image of a known size. Projector lenses describe the throw range and you need to be able to convert this into the expected image size. If not done correctly the image can be the wrong size or the projector may not work from the planned location.

PJ_Correct_Throw

Throw Ratio Too Low

PJ_Correct_Throw_1

Throw Ratio Too High

PJ_Too_Far_1

How is the Projector Lens Throw Distance Calculated?

Lens Throw Distance is simply the distance from the projector lens to the projected image In the image, the throw distance is 6.12m.

PJ_Correct_Calc

How is the Projector Lens Throw Ratio Calculated?

The lens throw ratio is the ratio of the lens throw distance to projected image width.

Example 1 - Calculating Throw Ratio from Throw Distance and Image Width

Throw Ratio (TR)

Throw Distance (TD) = 4.76m

Image Width (IW) = 2.8m

TR = TD:IW

TR = 4.76 : 2.8

TR = 1.7 : 1 (simply 4.76/2.8)

PJ_Throw_Example_1

Example 2 - Calculating Throw Distance Required 

TR = 1.7:1 
IW = 3.6m

TD = TR * IW 
TD = 1.7 * 3.6 
TD = 6.12m 

The projector would need to be 6.12m from the 3.6m wide screen if the lens throw ratio is 1.7:1

PJ_Throw_Example_2

Projector Lens Throw Consideration?

Lens throw calculations are normally quite straightforward but there are some important considerations that you should apply to avoid issues with your design. 

These calculations only apply when the screen aspect ratio matches the projected image aspect ratio from the projector. This can easily catch out the designer in systems where a DCI 4K resolution projector is being used on a 1.78 aspect screen. The 4096 x 2160 resolution actually has an aspect of 1.9:1 so is wider than the 1.78 screen. Without correcting for this the image may not be tall enough to fill the screen. 

No projector manufacturer gives an exact point where the measurement of throw distance should be referenced from. It is safest to use the distance from the face of the lens to the projected image. 

All projector lenses have some degree of variance in their performance vs their published specification. It is best to avoid being too close to the extreme of the throw ratio in case the lens is ever so slightly out of specification. If the lens throw range is given as 1.4 - 2.7:1 then it would be safest to position the projector within the range 1.5 - 2.6:1.

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