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What is keystoning?

When you tilt a projector up towards a screen the image projected becomes stretched, resembling a keystone. The same phenomena can be observed when a projector is tilted horiztontally relative to the screen or when tilted downwards from a ceiling mount. Edit

What is keystone correction?

To make the image projected appear square on screen when the projector is tilted, most projectors use keystone correction. This digitally alters the shape of the image projected. Some projectors do this automatically when tilted. Edit

Vertical vs horiztonal keystone correction

Most projectors feature vertical keystone correction, which makes the projected image appear square when the projector is tilted upwards or downwards. Some projectors also offer horiztonal keystone correction to make the image appear square when the projector is horizontally offset from centre of the screen. Edit

How does keystone correction compare to Lens Shift?

Lens Shift uses an expensive lens mechanism to move the position of the projected image around without shifting or tilting the projector. This maintains the quality of the image without noticeable loss of brightness or focus. More about Lens Shift.
By contrast, keystone correction digitally alters the projected image to appear square on screen. The lens does not move. Because the lens of a projector that uses keystone correction does not move, the focus and brightness of the projected image can vary across the screen when the projector is tilted. Also, because keystone correction alters the projected image it can reduce the quality of the projected image.
In classroom environments the use of keystone correction is generally not noticeable by your audience, which means Lens Shift is generally reserved for high cost high definition home theatre projectors and a limited number of high quality data projectors.