This is one question that is asked often, so here I am providing the brief explanation for users who are new to Canon DSLR system. The canon EF & EF-S lenses generally refer to mount type of lens that differentiates between its sensor type. At a time it is published, Canon provides 2 sensor types in the DSLRs –crop sensor or APS-C and full frame).
Brief Explanation on Full Frame & APS-C Cameras
Full frame sensor will be the larger of two. This is in size of 35 mm film frame – 36 mm x 24 mm. Canon APS-C is smaller over 22.3 mm x 14.9 mm. field of view is much smaller when using same lens on APS-C camera than it will be on the full frame camera.
Comparison of Full Frame & APS-C
To understand in detail about canon EF and EF-S lenses, it is important to know Canon EF 50 mm f/1.8 lens makes the view equivalent to 80 mm lens when it is used on the Canon APS-C camera like Canon Rebel line. It is man times referred as the “crop factor”.
Thus, Canon APS-C feature cameras have 1.6x crop factor. These APS-C cameras will include 7D line, 70D, lower line and Canon Rebel line.
The full frame Canon cameras include 1D X, 5D line and 6D line.
Canon EF and EF-S Lenses
In simple terms, Canon EF-S camera lenses are made completely for use on the APS-C DSLRs. And Canon EF camera lenses are made to work with the full frame as well as APS-C DSLRs from the Canon. The Canon EF-S camera lenses generally have the smaller image circle, which is big enough for covering smaller sensor that is found on the Canon APS-C cameras. Suppose Canon EF-S lens was used on the full frame DSLR, then it will produce heavy vignetting (corners will be black) because image circle formed by lens is very small to cover larger sensor. The Canon EF lenses are around since film SLR days. And they were made to cover 35mm frame. Because Canon EF lenses have the bigger image circle, they can cover the full frame sensors as well as APS-C sensors.
Advantages of Canon EF-S Lenses
Canon EF-S camera lenses are normally lighter and smaller compared to Canon EF lenses as less glass and smaller lens barrel will be needed to produce the equivalent view field for the lens that goes over APS-C camera, like Canon Rebel line, while compared to the full frame camera. In addition, EF-S lenses are normally affordable. Canon doesn’t produce any “professional” category of the EF-S lenses like pro “L” lenses that you will find on EF mount.