The focusing range can be selected by this switch to suit the shooting situation. This can reduce the time needed for autofocusing

The EF 300mm f/2.8L IS USM has three ranges:

2.5m - infinity (8.2ft - infinity)

2.5m - 6.4m (8.2ft to 21.0ft)

6.4m - infinity (21.0ft - infinity)

• Before switching the focusing range, turn the focusing ring to select a focusing distance somewhere within the desired range.
• If you do not set the lens to a focusing distance that is within the selected focusing range, autofocusing may stop when it reached the range boundary. This is not a fault. If it happens, press the shutter button halfway down again