An example image of the TS-E 24mm f3.5L. This is a tilt-shift lens - it allows one to either tilt the lens so it is no longer parallel to the sensor/film, or to shift the lens off axis but still parallel to the sensor/film.

The reason one would tilt the lens is to mess with the depth of field - the depth of field no longer is parallel to the film/sensor with the lens tilted.

In this example, the lens is tilted up. The focus is unchanged from the previous example, but as you can see, the depth of field is limited to the squares near the center. In fact, the knight on the right is mostly in focus from top to bottom - meaning that the depth of field is actually tilting toward the top of the frame.

One could use this effect, to create an artificially narrow depth of field in a landscape.

Or, rotating the lens so that the tilt is left-right, one could "cut" across the image (say) from front-left to rear-right, or vice versa.

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