Both are vertical angulation errors. Elongation makes the teeth look longer than they are; foreshortening makes them look shorter and stubby. They're opposites, and so are the fixes.
Which way to correct
- Elongation: the vertical angle was too flat (not enough). Increase the vertical angulation.
- Foreshortening: the vertical angle was too steep (too much). Decrease the vertical angulation.
Why the paralleling technique avoids this
With the paralleling technique, the receptor sits parallel to the long axis of the tooth and the beam is aimed perpendicular to both. If you use the holder correctly and align the PID with the ring, the geometry is handled for you. Elongation and foreshortening mostly show up when the receptor tips away from parallel (a shallow palate, a gagging patient) or when the bisecting technique is used and the angle is estimated by eye.
A good habit on every exposure: check that the receptor is parallel to the tooth, then that the PID is flush to the ring. Most distortion disappears when both are true.