What Does STF Mean When Talking About Photographic Lenses?
An STF, or Smooth Trans Focus lens is Sony’s name for a lens that uses an apodization element. In simple terms an apodization (also referred to as an APO or APD lens) is a type of neutral density filter which becomes thicker towards its edges. The gradual thickening of the element limits the amount of light allowed to transition the lens progressively towards the edges of the image area. This has the effect of smoothing the edges of out of focus objects within an image, creating beautifully soft Bokeh and an enhanced sense of depth. You can find more information in the glossary entry for Apodization Filter.