Sunday, 28 April 2013

Autofocus during video comparison, GH2 vs GH3

When the Panasonic GH1 was announced in April 2009, it had a unique selling point: It was the first and only consumer system camera which could autofocus continuously while recording videos. Since this time, the competition has improved a lot, of course, and all mirrorless system cameras can autofocus while recording videos. But they use different technologies, and the performance varies.

The Panasonic GH3 was released with a claim to have the best AF performance of mirrorless cameras ever, as usual for a new premium mirrorless camera. And the AF performance for single still images is very impressive indeed. However, this doesn't really matter. All current Micro Four Thirds camera focus more than fast enough for static still images. Except possibly with the Lumix G 20mm f/1.7 pancake lens, due to its combination of a fast aperture and an old school focus construction.

What's still a challenge, though, is continuous autofocus during video recording, and focus tracking of moving subjects, in AF-C mode.

The Sony SLT cameras solve this by using a fixed translucent mirror, which means being able to use phase difference autofocus (PDAF) also during video recording. This system is able to track moving subjects very well during video, due to the genuine SLR PDAF technology. However, the cameras are not mirrorless, being DSLR systems with fixed mirrors, which means having larger camera bodies, and, usually, larger lenses as well.

The Nikon 1 system and Canon EOS M system solve this by having on-chip PDAF sensors, directly on the imaging sensor. With this technology, they can combine PDAF and CDAF, however, the real world benefits of this system are still undecided.

Panasonic and Olympus have so far used pure CDAF, with no specialized hardware to aid the focusing. They rather rely on image processing to speed up the autofocus. With the Panasonic GH3 being the most recent premium model, let's see if it actually does improve upon the predecessor GH2. To test the cameras head to head, I mounted both on a plank using Manfrotto Superclamps:



On both cameras, I used the Lumix G 14mm f/2.5 pancake lens. The lens focuses very quickly. Even with the same lenses, the GH2 has a slightly wider field of view in video mode, due to the multi aspect sensor feature, which the GH3 misses.

Here are the results in terms of autofocus performance during video recording:



As we see, the Panasonic GH3 performs much better than the GH2 in term of autofocus. Even with the same basic technology, the GH3 has a better image processing capability, which enables it to focus better while recording videos.

Notice that the GH2 needs to jog the focus back and forth to confirm the focus and settle. This is a typical sign of CDAF focus technology. The GH3, on the other hand, appears to nail the focus straight away, as if it was using PDAF. Which it doesn't.

I think it looks like the GH3 is a revolution when it comes to continuous AF during video for Micro Four Thirds. It may be the first camera to make AF during video truly possible.

And this does work well in real life situations. Here, I have recorded a concert using the Olympus 45mm f/1.8 at f/2, ISO 3200. The light was very dim, around EV2. The autofocus was left on during the video, and it generally keeps the image well in focus. In my experience, the GH2 would not have handled such a situation well:



Keep in mind that AF-C while photographing moving subjects is a totally different subject. I would expect the Panasonic GH3 to perform better here as well, as it is capable of AF sampling at up to 240fps with the most recent lenses, the Lumix X 12-35mm f/2.8 and Lumix X 35-100mm f/2.8. However, I have not tested this feature yet.

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