The Sigma1.4x DG EX APO Tele Converterfor Pentax (also known as a tele-extender)
will multiply the focal length of the existing lens by 1.4x, and will
cut back the amount of light by one f/stop. For example, using a 1.4x
teleconverter with a 300 mm lens will effectively increase the focal
length to 420mm. A lens set to f/2.8 will now be f/4.
Sigma APO Tele Converter 1.4x and 2x are dedicated teleconverter
lenses. Due to the high optical quality of this teleconverter, this item
will only fit and work with a select list of lenses.