Projector Lens Shift
Making Installation Easier
If you had a chance to read over Digital Keystone Correction you will understand what happens to the projected image on the screen when the projector is not positioned exactly vertically and horizontally square to the screen. While Digital Keystone Correction corrects this issue digitally by compressing the picture, Lens Shift accomplishes through optics and as you will soon see is a much better solution.
What is Variable Lens Shift?
Lens shift was the first solution to remedy the need of having a projector positioned perpendicular to the screen. As the name implies Lens Shift is the actual mechanical physical shifting of the projector lens within the projector to square up the imager. Though this feature does add increased flexibility to how a presenter or home theater enthusiast postion their projector (usually within a 30° angle), thus they should keep in mind the degree of lens shift. Some projectors will correct (or shift) the image more so than others, so a little research might be required if you think you might be faced with extreme off-angle projections.
Lens shift provides space to move the lens itself left and right or up and down within the projector housing. This adjustment can be made either manually with a dial or joystick, or mechanically using the menu buttons. This extra space to reposition the lens means greater leeway in the placement of a projector in a room but does introduce added cost and a bit of fragility into designs.
A good example to help understand why a correction is required to fix “keystoneing”. Is if you take a flashlight and place it about 1-2′ feet from a flat wall, if you tilt it or change the angle to either the left/right or up/down you will notice how the image of the light projected changes shape as shown below. A projector with lens shift can make an adjustment to the off angle image without requiring you to physically move the projector. Rather, the lens inside the projector housing is moving to create the effect.
Keystone Correction Versus Lens Shift
There are several reasons why variable lens shift is better than its evil nemesis, digital keystone correction. This is because keystone correction is a result of the projector digitally correcting the image and then displaying this reformatted image. Because of this, image quality is lost during the process. Although it is a great feature, it is not recommend relying on it too heavily if you plan to use your projector to show video or use it in a home theater set up because the image undergoes a compression and conversion that recreates edge pixels which can cause noticeable artifacts in the corrected image.
Using your projector to display static text and images, using keystone correction in your projector is a great solution to distorted projections. You can read more about this our article about keystone correction.
Lens Shift Types
Some projectors have both vertical (up and down) and horizontal (left to right) lens shift. This means that the projector can be off axis to the screen both vertically and horizontally and the image can still be squared up. Although vertical lens shift is the most common type of lens shift, there is an increasing demand for horizontal lens shift. These adjustments can be accomplished either manually or through a power adjustment done with the onscreen menu.
Another recent feature that is worth mentioning - is projectors that offer variable lens shift to be shifted on a diagonal plane.
Double Up Projectors for Double the Lumens
Lens Shift corrects the picture without using Keystone Correction (which has its side affects). It also allows you to stack two similar projectors on top of each other, doubling the light output.
Home Theater Dilemma
People who purchase and install a screen or mount before purchasing their projector first often find themselves in a dilemma. They find they have limited choices in the projectors that fit their needs, because they need to fit a specific image size from a specific distance called the throw distance. That is why it is recommended to purchase your projector, 
and even your ceiling mount, before making a final selection on a screen. If you already have purchase and installed a screen before buying a projector, it might be a wise choice to research a projector that has variable lens shift. If the adjustments you require are extreme you potentially might require keystone correction done on top of the lens shift adjustments.