Sony's new STF (Smooth Trans Focus) 100mm F2.8 GM OSS uses an apodization (APD) filter to create very smooth out-of-focus regions/highlights both in front of, and behind, the focal plane.
But how exactly does it work? Sony has published a video demonstrating the principle, and the effect. Take a look:
The apodization element is a circular graduated neutral density filter that lets in more central light rays than peripheral light rays. This smooths out transitions between out-of-... more