When a projector is properly aligned with the screen the image will be “perfectly” rectangular. We all have been in rooms where the image was much wider at the top than the bottom, that trapezoidal shape is usually referred to as Keystoning.
Let’s start with why that happens. A projector typically sits on a table so that its lens is about even (or slightly below) the bottom of the screen. Since the distance to the top left and right corners of the screen is farther from the lens than the bottom left and right corners, the image spreads out more. The result: a wide image at the top and a narrow image at the bottom.
If you can’t move the projector, lens shift is the preferred way to correct keystoning issues because unlike electronic keystone correction, it doesn’t affect resolution. Lens shifting moves the lens itself up and down and left and right within the projector’s chassis to align the projected image to the screen. A projector may offer both horizontal and vertical lens shift but the amount of overall adjustment can vary by projector type due to various internal layouts of projectors. Normally 3LCD projectors have a larger range of available lens shift than a comparable DLP model. Also, the amount of vertical adjustments applied affect the amount of horizontal adjustment available.
Variable Lens Shift is the best way to keep your image rectangular unless you can place the projector right where it needs to be. For more information on Lens Shift check out the technical blog
we wrote a few years ago.