Autofocus & Performance

The Panasonic GX8 offers very similar AF performance to the Panasonic G7, which also features a 49-point Contrast Detect AF system and makes uses of Panasonic’s Depth from Defocus technology (when used with Panasonic's own lenses). Single autofocus performance, speed and accuracy is all top notch. And the GX8 has no problem using continuous autofocus at burst speeds of up to 6 fps. It can also subject track while firing bursts at 6 fps with a very impressive hit rate.

Although the camera can shoot at 8 fps, it drops to 6 fps if you use continuous AF. The latter is also the maximum speed of the camera's medium speed continuous drive mode (which continues to provide live view). We got our best results by switching to this mode - partly because it allowed us to see what the camera was trying to focus on (the high speed drive mode only shows you playback images between shots, so you don't get to see where the focus point is).

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The camera's AF tracking mode placed cross-hairs in the center of the screen; half-pressing the shutter prompts the camera to attempt to follow that subject around the frame. When using a Panasonic lens, the results are extremely good, especially for a system that doesn't have phase detection for depth measurement. The system can lose-track of its subject (seemingly if it gets too near the edge of the frame), but is generally very good at staying in focus and re-finding a subject if it loses it for a while. It's not as dependable as the systems on professional sports cameras (there are a few out-of-focus shots in the sequence above), but it's one of the more effective systems in its class. We also got solid hit rates when starting with a single AF point and using continuous autofocus.

Other AF methods

The GX8 offers both Touch-to-Focus and Touch-to-Shoot. And when one's eye is to the finder, the touchscreen can be used as an AF track pad. You can also set the four-way controller on the back to be an AF point selector (in the menu, turn ‘Direct Focus Area’ on).

Face Detect is a useful and well-implemented feature offered on the GX8. When a face is identified, a yellow square will briefly appear over it, followed by a green square indicating focus is locked. The camera has eye detection and will put a set of crosshairs over the eye it's locked onto. As you can see in the video above, this feature works well in both AF-S and AF-C, although, of course, results are lens dependent. You may experience a drop in hit-rate in continuous bursts - something most mirrorless cameras are prone to - but generally the camera didn't lose track of the face even at 6 fps.

For scenes with multiple faces, the GX8 prioritizes focus on the dominant face, which is usually the closest face to the camera. You can override the camera’s choice in dominant face using either the touchscreen or four-way controller by selecting an AF point over a different face of your choice. Unfortunately, this doesn’t tell the camera to prioritize the new face, it instead tells the camera to ignore Face Detect all together and prioritize the newly selected point only. We'd love to see a quick way to choose which face to prioritize, a la Sony's Eye AF.

Performance

We found overall camera performance to be excellent. The camera starts up are close to instant as possible, the refresh rates on the rear screen and EVF are incredibly high and the menu system and touchscreen interface are very responsive.

The GX8 has a number of burst modes, split up by which shutter you're using (mechanical or electronic) and whether you want single or continuous autofocus. Panasonic's claimed max burst numbers are 10 fps with the e-Shutter, 8 fps with mechanical and S-AF and 6 fps with either shutter type and C-AF. Do note that live view is not available at the 8 and 10 fps settings and that burst shooting with the e-Shutter may lead to rolling shutter if your subject is moving quickly.

To see if the GX8 measured up to Panasonic's claims we tested it in several modes using a SanDisk UHS-II Speed Class 3 SDXC card with a 280MB/sec write speed at a shutter speed of 1/320 sec.

High Speed - Single AF (claimed: 8 fps w/mechanical shutter, 10 fps w/electronic shutter)

Mechanical Electronic
Frame rate Buffer limit Frame rate Buffer limit
Raw + Large/Fine JPEG 8.4 fps 31 shots 10.1 fps 30 shots
Raw 8.5 fps 34 shots 10.0 fps 33 shots
Large/Fine JPEG 8.8 fps Unlimited 10.3 fps Unlimited

With autofocus locked, the GX8 meets or exceeds Panasonic's claims. Now let's see how it does in medium mode, which is the speed at which live view and continuous AF are available.

Medium Speed - Single vs Continuous AF (6 fps claimed)

Here we're going to take a look at how the camera performs at its medium speed, which is where gain both live view and support for continuous autofocus. As there is a ~1 fps performance hit when using image stabilization, we've turned it off for the purposes of this text.

Single AF Continuous AF
Frame rate Buffer limit Frame rate Buffer limit
Raw + Large/Fine JPEG 5.5 fps 33 shots 4.3 fps 33 shots
Raw 5.5 fps 41 shots 4.2 fps 41 shots
Large/Fine JPEG 5.5 fps Unlimited 4.4 fps Unlimited

As you can see, we were unable to hit Panasonic's claimed speed of 6 frames/second. There's also a 1 fps drop when using continuous AF, though that will vary depending on subject distance and movement.

Battery life

At 330 shots per charge (tested using the CIPA standard), the GX8 is on the lower end of the battery life spectrum among its mirrorless peers. The Olympus PEN-F gets the same number, while the Fujiflm X-Pro2 and Sony a6300 get 360 and 400 shots, respectively. Naturally, DSLRs will go for quite a while longer when shooting with their optical viewfinders, but will be be more compatible if using live view full-time. The GX8 does not support internal battery charging over USB (an external charger is included).