Field sharpness. From Doug Brown’s review it is clear that the extender 2X III delivers a crisp image with the EF 600mm F/4 IS II in controlled conditions and at close distance. A white-tailed kite is of course a more challenging subject as the soft, low-contrast white feathers expose any weakness in the optics, such as chromatic aberrations. Canon Extender Ef 2X ii International Version (No Warranty) 4.2 out of 5 stars 451 Auto Focus 2.0X Telephoto Lens for Canon, VILTROX TELEPLUS 2.0X Teleconverter Auto Focus Telephoto Extender Lens Converter for Canon EF Mount Telephoto Lens 70-200mm 100-400mm and DSLR Camera 80D 5DII This 25mm extension tube fits between camera body and the lens, providing even closer focusing than the Extension Tube EF 12 II. Compact and portable- a great accessory to take when shooting wildlife. Maintains electronic contact between lens and body. AED 777.00. The RF 400 f/2.8 weighs 6.4 lbs (2.9 kg). The 600 F/4, 6.8 lb (3.1kg). That's not a large weight difference. The 400 is 14.4" (367mm) long, while the 600 is 18.6" (472 mm) long. That's a more significant difference. They have almost the same entrance pupil diameter. Reply Reply with quote Reply to thread Complain. fyuDfXI.