This turns the camera on and off. There’s a further spring-loaded ‘lamp’ setting past the ‘On’ position that illuminates the control panel [06] on the top of the camera so that you can see the information in dark conditions. After a few seconds the camera will turn off the light metering system to preserve power – most of the information in the top control panel will disappear – but you can re-activate it by half-pressing the shutter button. You can save a little power by switching the camera off if you’re not using it for a few minutes, but DSLRs don’t use a lot of power in standby mode.