There are optical quality implications as well—not only because the image from the lens is effectively cropped—but because many lens designs are now optimized for sensors smaller than . The rear element of any SLR lens must have clearance for the camera's reflex mirror to move up when the shutter is released; with a wide-angle lens, this requires a retrofocus design, which is generally of inferior optical quality. Because a cropped-format sensor can have a smaller mirror, less clearance is needed, and some lenses, such as the EF-S lenses for the Canon APS-C sized bodies, are designed with a shorter back-focus distance; however, they cannot be used on bodies with larger sensors.