Smooth Trans Focus (STF) was invented by Minolta in the 1980s and became available in the Minolta 135mm f/2. 8 STF in 1999. The special design of the lens with an Apodization (APD) filter allows for the smoothing of out-of-focus areas, or bokeh.
The APD filter reduces the light transmitted through a lens, but the strength […]. petapixel.com