What is EF Mount?
This seems like such an easy answer. It is the mount that Canon cameras use these days. It is 44mm deep, it is capable of delivering power and information to a lens and transmitting information that allows the camera to control that lens. But those are the surface level answers. The truth is so much more interesting and nuanced.
The Canon EF mount was introduced in 1987, and is a gleaming example of a modern lens mount. Autofocus was unheard of until the early 80’s, when Minolta and others pioneered the technology. In the face of this revolution, companies generally took one of two approaches to integrating autofocus. Nikon chose to retrofit their existing lens mount to maintain backwards compatibility, while Canon simply stopped making FD lenses and started making EF cameras and lenses. This hard break allowed them to re-imagine what a lens mount should look like, and build a titan of this industry from the ground up.
But EF mount isn’t alone anymore- it is now the flagship of a growing family of Canon lens mounts. Canon has introduced EF-S lenses for APS-C cameras like the 7D Mark II and their consumer DSLRs, and EF-M lenses for their growing mirrorless lineup. Both of these types of cameras can accept EF lenses, but full-frame EF cameras cannot accept lenses from either of these two branches.
But Canon EF is also becoming a standard for midsize cinema cameras. These lenses cover a full-frame 35mm image, which is more than enough for the Super35 sensor size that has defined cinema cameras. The popularity of the 5D Mark II has created a generation of filmmakers that learned early on to invest in Canon lenses. In turn, that has incentivized manufacturers without their own proprietary mount to lean on Canon EF as a popular and accessible option. BlackMagic, Panasonic, and even RED cameras ship with EF mounts.
The Canon EF mount is so useful because it is so functional. It has a large diameter that easily mounts fat lenses. It is shallow - but not too shallow - and thus allows for many lenses to be adapted to it while also being adaptable onto other cameras. But most importantly, the lenses and cameras that natively carry the EF mount are beloved and high-quality. More than anything else, this engenders a loyalty that has lasted generations.