I've an older TCON 17 for 75 and worried about this screwed down rings on a Panasonic 100-300 for MFT. I no longer give the part of her. Swallows barely light and increases the focal length calculated on KB 1020mm in Aperture 5.6. The full-frame Canon Lens 1200 / 5.6 to about 100,000 (in words one hundred thousand) cost and weigh 16.5 kg. To each his own ...