The Smooth Trans Focus (STF) technology in photographic lenses uses an apodization filter to realize notably smooth bokeh with rounded out-of-focus highlights in both the foreground and background. This is accomplished by utilizing a concave neutral-gray tinted lens element next to the aperture blades as apodization filter, a technology originally invented (and patented) by Minolta in the 1980s, and first implemented in a commercially available lens in 1999. In contrast to soft focus lenses, STF lenses render a perfectly sharp image in the focus plane. Lenses featuring Smooth Trans Focus technology:
Minolta STF 135mm F2.8 [T4.5] (introduced 1999) Sony α STF 135mm F2.8 [T4.5] (SAL-135F28) (introduced 2006) Sony FE 100mm F2.8 STF GM OSS (SEL-100F28GM) (introduced 2017)
See also STF function (an emulation of the effect in the Minolta Maxxum 7) Fujinon XF 56mm F1.2 R APD (a similar lens introduced by Fujifilm in 2014) Venus Optics Laowa 105mm f/2 Smooth Trans Focus (a similar lens introduced by Venus Optics in 2016) f-stop T-stop
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