Digital King converters are designed to work with EF-S 18-55mm zoom lenses. They attach to the front of the lens using the 58mm filter mount.
This useful accessory screws to the front of an EF-S 18-55mm lens to give a 0.7x conversion. With the lens set to a focal length of 18mm, you get an effective focal length of 12.5mm. This gives a field-of-view equivalent to a 20mm lens on a full frame camera, bringing true super-wide angle focal lengths to your APS-C format camera. The alternative – an EF-S 10-22mm lens – costs over £900. No exposure increase is needed and autofocus is maintained.