The Thumb Rails are used for changing focus and exposure. These are primary camera controls and are as-such are always present in the viewfinder for easy access and use.
The focus Thumb Rail always controls the focus and will be visible if the chosen lens (hardware) supports manual focusing.
The exposure Thumb Rail controls either an exposure offset value (when in automatic exposure mode) or ISO (when in manual exposure mode). The exposure offset value works by automatically calculating the best ISO and shutter speed to achieve the desired exposure whereas the ISO control relies on the user understanding and setting up the shutter speed manually.
The Thumb Rails can placed in a left/right location; with the focus rail on the left and the exposure rail on the right. A left location, with both focus and exposure rails on the left, or a right location with both focus and exposure rails on the right. Being able to move the rails to either the left or right is very useful for one handed portrait filming
Thumb Rails Components
Rail
The rail is used to modify the value of the relevant camera control. It has been designed to rest under your thumbs (or fingers) whether you are filming in landscape or portrait. The Rail mimics a traditional camera control wheel and provides a fluid motion.
Icons
The icons shows what the rail is controlling. For focus the icons will display a rose and infinity symbols. For exposure they will show dim and bright sun symbols.
Near focus
Far focus
Reduce exposure
Increase exposure
Value readout
The value readout shows the current value for the control. For focus this will value between 0 and 1. For exposure it will either be an EO (exposure offset) value between -5 and +5 (when in automatic exposure mode) or an ISO value with a variable range (when in manual exposure mode)
Auto button
The auto button is a convenient button for jumping into auto or manual mode.
Tip: To quickly put the camera back into full auto and guarantee there are no manual controls active; tap the 'Full Auto' button from the Camera Control Wheel. Check out this support article for more information: Camera Control Wheel