Sony's STF Lens Achieves Smooth Bokeh with Graded Glass

Sony's 100mm f/2.8 STF lens incorporates a radially graded neutral density filter, known as an apodization element, to achieve exceptionally smooth out-of-focus blur. This element is clear in the center and darkens towards the edges, effectively softening the hard outlines of bokeh balls without affecting the in-focus image. The trade-off for this smooth blur is a reduction of approximately two stops of light, meaning the lens meters at f/5.6 despite having an f/2.8 aperture. This decades-old technique, first seen in Minolta lenses, remains a niche feature due to the significant light loss.