The original EF 600mm f/4 IS was a very sharp lens and it performed very well with the Canon EF extenders, including the latest EF 1.4X and 2X extender III. Despite great optics and the versatility to cover focal lengths from 600mm to 1200mm, the original EF 600mm f/4 IS had one significant disadvantage, its weight. EF 100–400mm f/4.5–5.6L IS II + Extender EF 1.4x III EF 200–400mm f/4L IS Extender 1.4x lens + Extender EF 2x III (built-in extender not used) If you are not using one of these combinations, you will be limited to a single central cross-type AF point. Canon FD Extender 2x-A Teleconverter Lens for FD Mount Made In Japan. C $67.09. OB Canon Extender EF 2x II Teleconverter w/ Pouch! Canon 2x Teleconverter. C $287.52. C $25.96 shipping. Compare Canon Extender EF 1.4X III vs Canon Extender RF 2x vs Canon Extender EF 2X III Press ‎ ↵ Enter ‎ for Accessibility for blind people who use screen readers Press ‎ ↵ Enter ‎ for Keyboard Navigation Canon Extender EF 2x III Tele Extender, USA 4.5 (268) $429.00 Canon Extender EF 1.4x III Tele Extender, USA Used Specials Canon Extender EF 2X III BH # RBC2XEF3 • MFR # 4410B006AA 587 reviews Authorized Dealer This image is for illustrative purposes only Key Features Refurbished Products Have a 90-Day B&H Refurbished Warranty EF Mount Teleconverter 2.0x Magnification Factor Microcomputer Controlled Show More Yours is a slightly different issue. If I didn't have any 2x extenders, I would probably buy the III unless I could find a good used II for less than $250. On the question of 1.4x vs. 2x, the image quality of my 1.4x is far superior to either the EF 2x or EF 2x III that I tested at comparable focal lengths. EF-S 60mm f/2.8 Macro USM; Extender EF 1.4x; Extender EF 1.4x II; Extender EF 1.4x III; Extender EF 2x; Extender EF 2x II; Extender EF 2x III; Extender RF 1.4x; Extender RF 2x; Mount Adapter EF-EOS R; Mount Adapter EF-EOS R 0.71x; Mount Adaptor EF-EOS M; MP-E 65mm f/2.8 1-5X Macro; RF 100-400mm F5.6-8 IS USM; RF 100-500mm F4.5-7.1 L IS USM; RF aFhUxD.