Canon ef mount !!!Great condition no issues. Can purchase an Mc-11 adapter to Sony e mount as well. From BH PHOTO: Presenting impressive reach in a relatively compact design, the canon ef 100-400mm f/5-6.3 DG DN OS Contemporary lens, from Sigma, is a versatile telephoto zoom mixing a hand-holdable design with apt optical performance. A series of low dispersion glass elements are used to achieve high clarity and color accuracy throughout the zoom range, and to suppress chromatic aberrations and color fringing. Complementing the optics is intuitive handling, in which a stepping motor delivers fast, near silent autofocus performance along with fine-tuned manual focus control. Additionally, four stop-effective optical image stabilization helps to reduce the appearance of camera shake for sharper handheld shooting and this 100-400mm is also compatible with the optional TS-111 Tripod Socket for steady shooting atop a tripod or monopod.