Similar to what happens with real-world cameras, Depth of Field (DOF) applies a blur to a scene based on the distance in front of, or behind, a focal point. The effect can be used to draw the viewer's attention to a specific subject of the shot based on depth and adds an aesthetic to make the rendering appear more like a photograph, or film.
Guassian
Gaussian DoF blurs the scene using a standard Guassian blur. The Guassian method is fast and generally well-suited for use in-game, where performance is critical.
Bokeh
As of Unreal Engine 4.23, Bokeh DOF has been removed from the Engine and source code. If using 4.22 and 4.21, Bokeh DOF has been deprecated for the Deferred Desktop Renderer and the Forward Rendering path.
Bokeh DOF refers to the name of the shape that can be seen in photos or movies when objects are out of focus. This method renders a textured quad for each pixel using a user-specified texture to define a shape with which to reproduce the effect of a camera lens.
The implementation only uses half-resolution to perform this intensive effect. It attempts to save render performance in areas where the effect is not noticeable by using Adaptive Depth of Field. Bokeh DOF is more costly than other DOF methods available in UE4, making this a prime candidate for cinematics and showcases where appealing visuals often outweigh any performance concerns in those situations.