Short Back Focus
From Wikipedia, the free encyclopedia
Short back focus is Canon Inc.'s descriptor for their EF-S lens. The distance between the rearmost surface of the lens and the sensor plane is shorter than that of a normal Canon EF series lens. A shorter back focus does have some advantages when designing wide-angle lenses, but there's a limit to how short it can be in an SLR lens since it can't be so short that the SLR mirror hits it when it flips up. The lower limit on the size of the mirror depends on the size of the film (or sensor). Medium format SLRs need a big mirror, 35 mm SLRs need a smaller mirror and SLRs with a digital sensor smaller than a full frame 35 mm frame can use an even smaller mirror. The "S" in EFS stands for "Short back focus".
Since the sensor in the Canon Digital Rebel is only 22.7 mm x 15.1 mm, smaller than the 24 mm x 36 mm full frame 35 mm frame size, the SLR mirror can be (and is) smaller, so it can use a lens with a shorter back focus than full frame cameras, hence the EF-S lens [1]
