Sony's new STF (Smooth Trans Focus) 100mm F2.8 GM OSS uses an apodization (APD) filter to create very smooth out-of-focus regions/highlights both in front of, and behind, the focal plane.

But how exactly does it work? Sony has published a video demonstrating the principle, and the effect. Take a look:

The apodization element is a circular graduated neutral density filter that lets in more central light rays than peripheral light rays. This smooths out transitions between out-of-focus elements, leading to quite unique imagery.

The APD element is a (circular) graduated neutral density filter inserted into the optical path of the lens. Out-of-focus light rays that are either converging in front of the focal plane, or diverging behind the focal plane, have a gradual radial softening (seen as darkening of the more oblique rays in front of or behind the focal plane). This yields less interference between out-of-focus light rays and, so, less 'busy' and simply smoother bokeh. 

What's the real-world impact? Have a look at the image comparison below, which compares the foreground and background bokeh with and without the APD element:

The APD element leads to smoother foreground and background bokeh. Note how out-of-focus highlights are smoothed, and this applies to everything. There's a cost though: sometimes I like sharp, enlarged de-blurred out-of-focus highlights, which you won't get with this lens. But what you will get is smooth, creamy bokeh.

We've had very little time with the lens, but our initial impressions of image quality are extremely positive. If you'd like to learn more about the lens, visit the company's dedicated page on this lens over at Sony Alpha Universe, then take a look at the phenomenal MTF curves on Sony.com's site. Yes, this is a sharp lens, but with beautiful bokeh. And image stabilization, to boot, which, combined with IBIS on most E-mount cameras, will allow you to use slow-ish shutter speeds to maximize light gathering, minimizing noise.

Which you'll need, because wide open, while you have the depth-of-field of F2.8, you have the light transmission of F5.6.

Sony's E-mount system is becoming increasingly hard to ignore for professional results. With the release of the 100mm STF GM and 85/1.8 lenses, Sony is rounding out a format that already accepts arguably the largest lens lineup in history (thanks to its short flange distance). But the importance of a native lineup cannot be over-stressed, as it is native lenses that benefit most from Sony's AF technologies.

A note on autofocus...

Similar STF lenses tend to be manual focus or contrast detect-only, since peripheral light rays - the very ones the filter is designed to block out - are necessary for traditional phase-detect sensors to function. However, since (at least Sony's) on-sensor PDAF sensors can still use more central light rays for focusing, focusing does still work, remarkably well in fact with the a7R II, even indoors, wide open. That's quite an achievement, in no small part due to the excellent focusing system of the a7R II. What's more, movement of the focus element(s) is incredibly fast, thanks to the excellent Direct Drive SSM mechanism we've seen in blazingly fast-to-focus lenses like the FE 35mm F1.4 and FE 24-70mm F2.8 GM.

"Unlike Sony's recent releases, this lens focuses wide open..."

In other words, I was able to use to the 100mm STF to focus on faces, even using Eye AF, in fairly low indoor lighting, despite the T5.6 rating at a F2.8 aperture. This is in no small part due to the fact that this lens, unlike Sony's recent releases, focuses wide open (albeit stopping down after initial acquisition in AF-C). More tests to follow, but simply the fact they got phase-detect AF working, nevermind the AF performance, is nothing short of impressive.

The FE 100mm F2.8 STF GM OSS is already shaping up to be a spectacular lens, adding to an already well-rounded, serious E-mount lens lineup.