EF vs. EF-S

Today, in addition to its EF lenses, Canon offers lenses that use the EF-S (the S stands for “short back focus”) mount, with the chief difference being (as you might expect) lens components that extend farther back into the camera body of some of Canon’s latest digital cameras (specifically those with smaller than full-frame sensors), such as the XT and XTi. As I’ll explain next, this refinement allows designing more compact, less-expensive lenses especially for those cameras, but which don’t fit models like the EOS 5D, 1Ds Mark II, or 1D Mark III and 1D Mark II N (even though the latter two cameras do have a sensor that is smaller than full frame).

Canon’s EF-S lens mount variation was born in 2003, when the company virtually invented the consumer-oriented digital SLR category by introducing the original EOS 300D/Digital Rebel, a dSLR that cost less than $1,000 with lens at a time when all other interchangeable lens digital cameras (including the XTi’s “grandparent,” the original EOS 10D) were priced closer to $2,000 with a basic lens. Like the EOS 10D introduced earlier that same year, the Digital Rebel featured a smaller than full-frame sensor with a 1.6X crop factor (Canon calls this format APS-C). But the Digital Rebel accepted lenses that took advantage of the shorter mirror found in APS-C cameras, with elements of shorter focal length lenses (wide angles), that extended into the camera, space that was off limits in other models because the mirror passed through that territory as it flipped up to expose the shutter and sensor.