This Tele Converter will multiply the focal lenght 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. Note: Due to the high optical quality of this teleconverter, this item will only fit and work with a select list of lenses. Please click here for a compatibility chart of all compatible lenses.