Amazon.com : VILTROX C-AF 2X II TELEPLUS 2.0X Teleconverter Auto Focus Telephoto Extender Lens Converter for Canon EF Mount Super Telephoto Lens 70-200mm 100-400mm and DSLR Camera 80D 5DII : Electronics Re: Canon Extender 1.4x III with Sigma 150-600. In reply to John Sheehy • Aug 10, 2021. My answer still stands, the 5d4, tc 1.4 v3 and the 100-400 give poorer results than the straight 5d4 and the 100-400 then cropped. Yes, the combination does tracking flying objects and they arena focus but lack detail. Two things: First, a tele-extender not only changes the focal length by its multiplier factor, but ALSO the aperture by the multiplier factor. That means 300mm and f/5.6 with a 2x becomes 600mm and f/11; with a 1.4x it becomes a 420mm and f/8. Your camera's autofocus system will not work correctly at f/8 nor f/11 -- it's not enough light. Thanks again for your researched and thoughtful replies. I'm convinced I'll just stick with the 400 mm until (if ever) I can afford a new generation super telephoto. The reduction in light passing through the lens, is similar to stopping down the aperture, or reducing the size of the lens aperture. A 1.4x extender changes the effective focal length of a 100mm f/4 lens into a 140mm f/5.6 lens. Notice how both the focal length and f/stop have been multiplied by 1.4. 1. 2X III works really well and maintain a reasonable sharpness only on Superteles, 100-400 II and on on 70-200 II and unfortunately not included the 100-400 I. It's a waste of money. kingfishers are one of the skittish creatures, it posts a challenge even with 600mm + 2x. If you use a 2x with 400, you may find the distance not usable as you A concise review and overview of the Canon EF 2X III Extender. My website: The Canon EF 2x II Extender is an easy and very affordable way to dramatically extend the reach (focal length) of compatible Canon L Series lenses (and a few additional lenses). This review will sound very similar to the Canon 1.4x Extender Review, but the 2x function is basically the same. The Canon EF 2x II Extender mounts between the flxvg.