Autofocus

Fujifilm GF 110mm F2 | ISO 1250 | 1/500 sec | F4
Photo by Wenmei Hill

Key Takeaways

  • Autofocus is flexible and fast by medium format standards (but behind contemporary full frame performance levels)
  • Focus is highly accurate and more than precise enough to rely on
  • The challenges of driving large lenses quickly blunts AF-C performance
  • Eye AF frees you up to think about composition but isn't always accurate enough (and is not reliable enough with multiple subjects)

The GFX100 is the first of Fujifilm's medium format cameras to have phase detection elements on its sensor. Fujifilm was a pioneer of this technology, which provides the autofocus system with a degree of distance information. This allows it to assess which direction and how far it need to drive the focus mechanism.

This immediately offers a speed benefit over the existing GFX models. However, this is only part of the story: the speed at which the lenses can be driven is another important factor in focus speed. The GF lenses are somewhat variable in this sense: they have a variety of designs and use different types of focus motors. They're all significantly faster than the autofocus lenses for other medium format systems (such as Phase One or Hasselblad's X system), but not as snappy as the best contemporary mirrorless systems.

Focus Precision and Consistency

Single AF is generally fast and appears to include a contrast-detection hunt as it nears the point of focus. This makes sense for trying to maximize accuracy and precision on such a demanding camera.

Focus precision

100MP lets you see every fractional change in focus between shots in incredible detail. Shooting ten shots back-to-back we could see slight imprecision, shot-to-shot, but only by obsessively scrutinizing images side-by-side at 100%. We do not believe these differences have any real-world consequence. Example 1 below is the strongest result from those ten images, and example 2 is the weakest.

Focus drift

If you choose to manual focus (either directly or via tethering) we found that focus would drift away from the specified position over time, even without any physical contact with the lens. Again the differences were very small and will only be an issue if you need to shoot large numbers of shots with focus fixed in exactly the same position.

Continuous Focus and Focus Tracking

Clearly the GFX100 isn't designed to be a sports camera. However, part of its purpose is clearly to extend the range of environments in which medium format can be shot, so we feel it makes sense to at least look at how its continuous AF and focus tracking perform.

In this first test we shoot using the central AF point to test the camera's ability to judge the distance of an approaching subject and to drive the lens to that distance. In this instance we used the GF 250mm F4 R LM OIS WR, to match our standard 200mm-equivalent focal length.

As you can see, it pretty very well. It's not perfectly focused in every shot but it's extremely close, given how closely a 1:1 view is letting us view these images. That said, the camera only performs AF-C in its 3fps mode and we found that this was dropping to nearer 1.3fps to get these results.

We then try the camera's tracking mode, where it not only has to judge depth but also needs to recognize and follow the subject as it moves around the scene.

Subject tracking doesn't perform as well. In every attempt (with multiple settings), the camera lost its subject and refocused to the background.

Despite borrowing the configuration options from the X-T3, the GFX100 doesn't show anything like the same performance. We tried multiple settings but found the camera lost track of the subject every time and reverted to focusing on the background.

Even so, this is a stress test for a camera like this, and its ability to refocus in its basic focus modes is pretty impressive, relative to other medium format cameras.

That said, focus hunting can be pronounced, at times and gets worse as you stop the lens down, or as light levels fall.

Face / Eye AF

The GFX100 has a version of the Eye AF system developed for the X-Series cameras. This can focus on the eye and lets you use either the joystick or the touchscreen to select between eyes that the camera has detected.

The differences in light levels (and hence ISO setting and level of shake) mean these images are not as directly comparable as we'd like, but the difference in focus position should be apparent

The GFX does a reasonable, but not great, job of detecting and maintaining its awareness of faces. It's also a little prone to false-positives: finding faces in unexpected parts of the background scene. This significantly undermines the ability to choose between faces, since the camera can lose track of the face you're trying to select. We certainly wouldn't rely on this system during a fast-changing situation, such as candid shots at a wedding.

A dependable Eye AF system can free you up to concentrate on composition and your subject

Even with posed portraiture with just a single subject, we weren't especially impressed with where the camera tried to place the focus. It regularly focused on the subjects' eyelashes, rather than the eye itself, and we were able to get better results by using a single very fine AF point, or manually focusing.

This mis-targeting will be less noticeable when your shots have greater depth of field, but still means the point of peak focus will be in the wrong place. This is a pity, as a dependable Eye AF system can free you up to concentrate on composition and interacting with your subject.