The Bokeh Master
Delivering both smooth bokeh and fine sharpness, the FE 100mm f/2.8 STF GM OSS from Sony is a short-telephoto prime featuring a unique and sophisticated optical design. Separating itself from other portrait-length lenses, this 100mm f/2.8 features Smooth Trans Focus (STF) technology, which uses an apodization filter to realize notably smooth bokeh with rounded out-of-focus highlights in both the foreground and background.
 
Smooth Trans Focus
Optical design incorporates an apodization (APD) element that helps to improve the quality of bokeh. This element, which resembles a radially graduated ND filter that tapers from clear in the center to denser around the edges, produces more circular out-of-focus highlights for more pleasing selective focus and shallow depth of field effects.
An aperture range of f/2.8 to f/20 is available, however the implementation of the apodization filter renders this range as T5.6 to T22.
A rounded 11-blade diaphragm further contributes to a pleasing bokeh quality when employing shallow depth of field techniques.