Progressive refinement
Overview
In the SFM, a single frame is not a still image; it's 1/x of a second (x being the number of frames per second your project is working in, default being 24). This means that any object in motion in the frame is in a slightly different position at the end of the frame than it was at the beginning. That motion data is stored in your session.
Progressive refinement is a rendering technique that creates motion blur, depth of field, and soft shadows by blending together slightly different versions of a frame many times. The more versions that are blended together, the higher the quality, but the longer the movie takes to render.
Note: Progressive refinement is disabled when using the Motion Editor, Graph Editor, and the work camera. You won't see progressive refinement when playing back a scene. To view a frame with progressive refinement, ensure your viewport is set to your scene camera, you're in the clip editor, and you've paused playback.Sample Settings
Currently, the amount of samples you can utilize towards progressive refinement are as follows:
Depth of Field: 8, 16, 32, 64, 128, 256, 512, 1024
Motion Blur: 8, 16, 32, 64, 128, 256
Note: You may also use your camera settings to render progressive refinement, as selecting any other sample value shall override your current camera settings. You can edit the camera's values in the element viewer.