Photo nerd - the bit of gear they use to turn a 400mm lens into an 800mm lens is called a 2x teleconverter. You get double the focal length for a reduction of t

Photo nerd - the bit of gear they use to turn a 400mm lens into an 800mm lens is called a 2x teleconverter. You get double the focal length for a reduction of two stops in the available aperture and generally a loss of optical resolution.