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

The EF 600mm f/4L IS USM has three ranges:

5.5m - infinity (18.0ft - infinity)

5.5 - 16.2m (18.0ft to 53.2ft)

16.2m - infinity (53.2ft - 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