A strong magnetic field is generated by passing a current through a set of windings. This field acts as a convex lens, bringing off axis rays back to focus. Click on 'Draw rays' to compare the action of an electromagnetic lens with an optical lens.

The image is rotated, to a degree that depends on the strength of the lens. Focal length can be altered by changing the strength of the current.

More detail on how magnetic fields affect electrons.

How can we change the focal length of a glass lens in a light microscope?