Digital single lens reflex cameras (DSLRs) come with a variety of sensors, the actual chips that capture the image—the "film" within a digital camera. Top-of-the-line models generally use sensors the same size as 35mm film ("full-frame sensors"), which allow the digital camera to capture everything a film camera would capture using the same lens and zoom factor.
Most DSLRs, however, use a smaller (and less expensive) sensor. Using the same lens and zoom factor, these cameras capture less of the scene. However, special lenses, made with these smaller sensors in mind, are available.
Do they actually make a difference? Let’s compare, using a couple of Canon EF-S lenses (designed for smaller sensors) with Canon EF lenses (for use with all Canon SLRs, film and digital).
Physical Differences Between EF-S and EF Lenses
The first thing you should note about Canon’s EF-S series lenses is that they can be used only with specific models of DSLRs. The Canon Rebel and Rebel XTi, 20D, 30D, and 40D comprise the current lineup for these lenses.
As you can see in Figure 1, the lenses are physically different, with the –S series lens (on the right) extending a bit farther into the camera body. This makes the –S series lenses incompatible with full-frame cameras, such as the Canon 5D. (Canon wisely makes it impossible to mount an –S series lens on a camera for which it wasn’t designed, preventing accidental damage to the lens or the camera’s mirror.)
Figure 1 The –S series lenses are physically not compatible with Canon cameras using a full-frame sensor.
There’s another important physical difference between the EF series lenses and the EF-S series lenses: the actual size of the glass within the lenses. As you can see in Figure 2, the last element in the EF–S series lenses (the pair on the right) is substantially smaller than those of the four EF lenses on the left.
Figure 2 Compare the two lenses on the right (EF-S) to the four on the left (EF).
By bringing the final element of the lens closer to the sensor (Figure 1) and reducing the image circle to a size more appropriate for the APS-C sized sensor (Figure 2), the EF-S series lenses provide a bit wider angle than would the same lens elements arranged in a standard Canon EF lens. The EF-S series, according to Canon, also reduces the size, weight, and cost of wide-angle lenses for these cameras.