The exposure controls let you view detail in overexposed lights or in the shadows of environment maps. It does not affect the brightness of any of the lighting itself, it changes the exposure level of the image being displayed on the screen.
Exposure Control
Click and drag on the button, right and left to increase or decrease the exposure or type values into the text field. Double click on the icon to reset the value to 1.
Example of adjusting the Exposure of the Canvas View