Ambient Occlusion

Valve's Source engine uses SSAO which stands for Screen Space Ambient Occlusion.
What Visual Difference Ambient Occlusion Makes
Enabling Ambient Occlusion
Enabling Ambient Occlusion is as easy as right-clicking the viewport and heading to your render settings. In the dialog box for your render settings, check the box adjacent from the Ambient Occlusion setting which reads "enable". Make sure that the drop-down menu next to the AO setting reads "AO Only" and not "Outline(link) Only" or "AO & Outline" (edit for proof, add pictures).

-Comparison picture: No DoF versus DoF at varying sample rates
Camera Settings
SSAO Bias: Determines the threshold for visible ambient occlusion
SSAO Strength: Determines the intensity of the shadows cast by ambient occlusion
SSAO Radius: Determines the filtersize (spread) of shadows cast by ambient occlusion
