Sony's G Master lens family has a new member in the form of the FE 100mm F2.8 STF GM OSS ($1,499.99). Like other GM lenses, it's designed for the high-resolution image sensors of today and tomorrow, but the 100mm F2.8 STF has a special trick up its sleeve. An integral apodization element cuts incoming light, smoothing out-of-focus highlights for a more pleasant background. Add in a close-focus mode that delivers 1:4 magnification and you get a versatile, albeit expensive, prime lens. It delivers extremely sharp images, but its light-gathering capability is compromised by the apodization element. It's a powerful tool for portraiture, but not the best choice to use in dim light.

Design

The 100mm F2.8 STF ($1,498.00 at Amazon) measures 4.7 by 3.4 inches (HD), weighs 1.5 pounds, and supports 72mm front filters. It's large compared with other lenses of this focal length, but balances well on full-frame Sony mirrorless bodies. Its barrel is tough polycarbonate, with a matching lens hood that attaches via bayonet and reverses for storage.

Like other FE lenses and Sony's full-frame mirrorless camera series, the 100mm is protected from dust and moisture. There are three control rings: manual focus sits right behind the front element, the aperture control and STF control ring is at the middle of the barrel, and a ring near the base changes between the standard and close focus ranges.

There's also a Focus Hold button that locks the autofocus system when held down. It can be reconfigured. I usually set it to activate Sony's Eye AF system, which locks focus on your subject's eyes, which is the preferred focal point for portraiture. The lens has three switches: an AF/MF toggle, a switch to enable or disable the in-lens optical stabilization system, and a mechanical control to switch between clicked or declicked operation of the physical aperture ring. Videographers will want to go with declicked operation, as it doesn't introduce as much noise to the soundtrack and allows for smooth adjustment of the diaphragm.

The 100mm F2.8 focuses as close as 2.8 feet (0.85-meter) in its standard focus range. Twisting the rear dial extends the lens elements forward, cutting the minimum distance to 1.9 feet (0.57-meter), but giving up the ability to focus to infinity. When working at the minimum focus distance, the lens projects objects onto the sensor at 1:4 life-size magnification.

Smooth Trans Focus

The big selling point of the FE 100mm is its apodization element, which delivers what Sony calls its Smooth Trans Focus capability. This isn't the only lens out there with this type of element. The Fujifilm Fujinon XF 56mm f/1.2 R APD, which won't work with Sony cameras, and the manual focus-only Venus Optics Laowa 105mm f/2 (t/3.2) Smooth Trans Focus, which is available in a version for Sony mirrorless cameras, both offer similar functionality. But the FE 100mm is the only lens of its type that covers a full-frame image sensor and offers autofocus.

The APD element is internal, and is essentially a neutral density filter that is stronger at the edges than at the center. It cuts more light away at the edges of the lens compared with the center. The result is an out-of-focus area behind your subject that is more smoothly blurred than a similar lens that doesn't have the same internal filter. To accentuate the quality of the out-of-focus blur—the bokeh—the FE 100mm has an aperture with 11 blades, so highlights remain circular at narrower apertures. The aperture and APD filter work together to eliminate the cat-eye shape that highlights often form toward the edge of the frame. Instead you get perfect circles from edge to edge, as you can see in the uncropped image above.

Of course, because the amount of incoming light is cut, the f/2.8 lens doesn't gather as much as other f/2.8 lenses. The aperture ring is marked in f-stops from f/8 through f/16, but wider settings are marked in T-stops. A T-stop, more familiar to cinematographers than to photographers, measures the transmission of light rather than the size of the opening through which it passes.

At f/2.8, the 100mm STF gathers as much light as a typical f/5.6 lens, and the APD filter gives backgrounds the look of an f/2 or f/1.4 lens, while maintaining the depth of focus of f/2.8. That's a lot of numbers, but basically you should know that you'll need to throw more light on subjects than you would with a standard f/2.8 prime in order to properly expose a scene. To help you calculate exposure manually, the camera body displays the T-stop value through settings from f/2.8 (t/5.6) through f/5.6 (t/7.1). At f/8 through f/22, the t-stop and f-stop values are close enough not to require such a distinction.

If you often work in natural light and want a portrait lens that can excel in dim conditions, you'll be better served with a brighter one. The FE 85mm F1.4 GM is the best we've tested for the Sony system.

Image Quality

The proof is in the pudding, or in this case the photos. I had a couple of hours to use the 100mm STF in a controlled studio environment, with scenes set up to provide out-of-focus highlights that really show what the lens can do. I also had the FE 85mm F1.8 on hand, and was able to capture similar scenes with both lenses. Take a look at the two images below—the photo on the left was captured with the 100mm STF, and the shot on the right with the FE 85mm F1.8. Both foreground and background blur are smoother, more circular, and have pleasing feathered edges when shot with the STF lens.

I tested the FE 100mm with our standard SFRPlus test chart and Imatest. At f/2.8, marked as t/5.6 on the barrel and for some reason displayed as f/5.6 in the camera, the lens notches an incredible 4,290 lines per picture height. That's much better than the 2,200 lines we want to see at a minimum, and while the edges of the frame lag behind the center, they're still crisp at 3,875 lines.

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There's modest improvement at t/6.3 (4,310 lines) and t/7.1 (4,421 lines). Diffraction sets in a bit early, cutting resolution to 4,251 lines at t/8 and 4,240 lines at t/11, before taking a more noticeable toll at t/16 (4,004 lines) and t/22 (3,301 lines).

Even though it's a G Master lens, the optics aren't completely perfect. There is about 1.5 percent pincushion distortion visible in photos. This type of distortion adds an inward bow to straight lines. It's noticeable in lab test images, but I didn't find it overly distracting when photographing people with the lens. But if you're shooting a portrait with some architecture as the background of the scene, you may find the need to correct it via software. We also test lenses to see how even illumination is from center to corner. The FE 100mm is very strong in that regard, capturing photos that are almost perfectly evenly lit across the frame. If you want a vignette around a portrait, you'll have to add it with software.

Conclusions

The Sony FE 100mm F2.8 STF GM OSS is a lovely lens for shooting portraits. In a studio or daylight the limited light-gathering capability isn't a liability, but at f/5.6 you're going to need to push your ISO or use a flash for most indoor environments. Wedding photographers should take a close look, as the lens is a serious tool for capturing environmental portraits of the happy couple, especially if you're shooting in an area where the background may get a bit busy. The differences in rendering may be a bit subtle—it's a lot easier to appreciate the quality of the bokeh when looking at images shot side-by-side with other lenses—but any Sony shooter with a strong penchant for portraits should consider adding the FE 100mm to their kit.

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