Part 1. What is keystoning? Projector lenses are made to shoot an image above the actual projector. This enables an audience to see the image without the projector itself getting in the line of sight. For example, in figure 1, the projector is lying flat and is not tilted at all.
If projector lenses were not made with "fixed keystone correction", the projector's image would land both on and off of the screen, as seen in Figure 2.
When a projector is not set perfectly perpendicular to the screen, it will cause a distortion of the image. For example, the projector in Figure 3 is tilted upward.
Figure 4 shows the trapezoidal image that caused by the projector being tilted upward as shown in Figure 3.
The resulting trapezoidal image is referred to as "keystoning," or a keystone problem. Fixed keystone correction is the ability of most projectors to shoot upwards, which makes tilting the projector upward less necessary. If fixed keystone correction is listed on a specification sheet, it will give the angle degree that the image shoots upward.
Digital keystone correction is a function that most, if not all, projectors have that enables the user to digitally correct keystone problems when the end user is forced to tilt the projector further than the fixed keystone correction allows for. (Shown in Figure 3 and Figure 4).
Digital keystone correction is a function of the projector's scaler. The projector resizes the image so that the image will appear to become square again as shown in Figure 5.
This image manipulation is called vertical keystone correction. Vertical keystone correction also works if the reverse has happened and you need to tilt your projector downward instead of upward.
While all projectors now have vertical keystone correction, horizontal keystone correction is much less common.
Horizontal keystone correction is needed when the front face of the projector cannot be placed parallel to the screen surface as seen in Figure 6.
Horizontal keystone correction will correct the image the image as shown in Figure 7.
As a final note, we strongly recommend not using any type of digital keystone correction if it at all possible. Because digital keystone correction is a function of the projector's scaler, it can and will produce an image with slight visual distortions and artifacts. Setting up your home theater correctly from conception negates any need for keystone correction.