Defines how the active camera will "see" the flare. You can use this property page if a scene has several cameras but you do not want all of them to render the flare effect.
To Display: Expand the camera node in the explorer view and click the Flare Properties icon (will only be visible if a flare has been applied).
Lens Guard Margin: Defines a rectangle around the view plane with width equal to margin. The flare intensity falls off linearly within this region. This parameter blocks out direct rays from light sources outside the view angle. In screen space, this means that light(s) outside the view area should not cause lens effects.