The lens shift describes how far in percentage screen dimensions, the projected image can be moved from the on axis position (normally projector exactly in the middle of the screen) without requiring the projector to be physically moved.
What is Projector Lens Shift?
Why is Projector Lens Shift Important?
In most cases, the goal is to install the projector in a location which will not be in the line of sight of the viewer or otherwise inconveniently located in the room. Projector lens shift allows the image to move relative to the projector which allows the projector to be installed in a more convenient location without impacting the image quality.
On Axis
Lens Shift Down
Lens Shift Up
How is the Projector Lens Shift 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.
How is the Projector Lens Throw Ratio Calculated?
The lens shift is expressed in percentages of screen dimensions. These values are provided in the projector lens specification. This seems a strange number but actually makes a lot of sense.Example 1 - Projector On Axis (in line with the exact middle of the screen)
This is 0% lens shift vertically and 0% lens shift horizontally.
Note that 50% of the image is above the projector and 50% of the image is below the projector
Example 2 - Projector level with top of screen
This is 50% lens shift vertically and 0% lens shift horizontally
Note that 100% of the image is now below the projector so it has shifted vertically down by 50%
How is the Required Projector Lens Shift Calculated?
To calculate the required lens shift needed to install the projector in the correct location, we can calculate the actual distance values then convert to percentage screen dimensions.Example 1 - Calculating the shift required if the projector and screen location are fixed
Projector Install Height (PIH) = 2.2m (middle of projector lens)
Screen Install Height (SIH) = 1m (middle of projector screen)
Screen Width (SW) = 3m
Screen Aspect (SA) = 1.78:1
Screen Height (SH) = SW/SA = 3/1.78 = 1.69m
Shift Distance = PIH-SIH = 2.2m - 1m = 1.2m
Percentage = 100 (1.2 / 1.69) = 100 * (0.71) = 71% vertical lens shift required
Projector Lens Shift Considerations
It is really important to calculate the lens shift correctly so that the projector is installed in a valid location. It is all too common to see the projector tilted forward due to an incorrect calculation. This causes an optical error called Keystone but can also result in uneven focus between the top and bottom of the screen.Most home cinema projectors have a lens shift of 50-60% which allows them to be installed in line, or just above the top of the screen. It is possible to find models with much more lens shift than this - but that implies the projector will be using large and relatively expensive lenses.
It is not possible to use all of the horizontal and all of the vertical lens shift simultaneously. The lens shift is limited by the image fitting on the lens so it is common that 100% vertical shift allows 0% horizontal shift.
It is expected that the horizontal shift percentage of the lens will be less than the vertical shift percentage. This is simply because most projectors have an aspect ratio with a width much greater than the height. In a normal 1.78 projector, a vertical lens shift of 50% is equivalent to a horizontal lens shift of 28%.