The Smooth Trans Focus (STF) is all about bokeh — or those out-of-focus lights that turn into abstract floating circles in the background (and sometimes foreground) of an image. Getting bokeh with a wide f2.8 lens isn’t hard to do, but STF is all about the quality of that bokeh.
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The SFT lens uses one extra piece over a typical lens. That piece is an apodization element that blocks out more of the light rays on the outside than toward the middle. Essentially, the apodization works like a graduated neutral density filter, only instead of a linear shape, the filter is a radial one that’s darker towards the edges.
That sort of built-in filter changes the way that bokeh looks. Instead of a circle with hard edges, the SFT lens produces a circle with soft edges that fade out. Those bokeh circles also appear to have less overlap.
Because of where the apodization element is in the lens, it affects the out-of-focus light rays so that what’s in focus, the subject, still has sharp edges.
While the subject stays sharp, there is a trade-off. That extra piece limits the amount of light that reaches the camera’s sensor, since it is filtering out some of that light to achieve the smoother bokeh. So, when shooting with the lens wide open at f/2.8, it’s the equivalent to shooting at an f/5.6 in terms of exposure. Essentially, that means you get the bokeh of an f/2.8 with smoother edges, but the low-light performance is of an f/5.6 lens.
The SFT element on Sony’s latest pro-level lens creates a different look to the background bokeh, though the trade-off is less light coming through the lens. The Sony 100mm f/2.8 GM STF GM OSS
