In order for Element 3D's depth of field to work, you must turn ON Depth of Field on your scene camera.
Element features several modes for generating Depth of field and one mode that visualizes the focus plane with a red highlight.
This mode renders multiple passes of your scene in order to generate 3D depth of field that most accurately matches the AE Camera Blur amount. However it is also the slowest for complicated scenes. Increasing the quality will increase the render passes exponentially. It is recommended to use the lowest quality that produces adequate results. NOTE: multi-pass renders your entire scene so using multi-pass depth of field with motion blur will slow down rendering dramatically.
This mode uses a post processing lens blur that works well and renders consistently. Meaning the speed of the blur is not dramatically affected by the complexity of the 3D scene unlike the Multi-Pass mode which requires repetitive rendering of the scene.
These modes are very fast but not accurate, and they can create some artifacts on small objects. Great for testing and using in low featured 3D objects or surfaces that do not have a lot of detailed mesh.
This is a visual representation of the depth of field that shows your focal plane represented by a red tint. The more red, the more in focus.