Monday, 1 April 2013

AF during video, comparison GH2 vs GH3

Recent Micro Four Thirds cameras have very good autofocus performance for still images. Mostly, the performance is among the best in this class, certainly better than DSLR cameras in live view mode. However, there is one area where mirrorless cameras don't perform well at the moment, and that is continuous autofocus: Both during video recording, and for photographing moving objects, e.g., for photographing sports and birds.

Some camera manufacturers have been trying to solve this by adding phase difference sensors (PDAF) on the imaging chip, like the Nikon 1 and Canon EOS M cameras. However, the real world benefit of that solution is still undecided.

Panasonic have said in interviews that the on-sensor PDAF solution is not going to be used for Micro Four Thirds, at least not anytime soon. Rather, Panasonic expects to achieve better continuous autofocus performance by using faster image readout from the chip, better image processing algorithms, and more processing power. Have they achieved this with the most recent Panasonic GH3?

To compare the autofocus performance during video recording with the GH2, I used a Lego Technic contraption to move a cardboard box back and forth at a steady pace. I then set up both cameras, in turn, with the Lumix X 12-35mm f/2.8 lens at 35mm f/2.8 at close range, and recorded video at 1920x1080, progressive, 25fps. Comparing the resulting footage, it is easy to see which camera better finds the focus during the movement. Here is the video footage, for comparison:



It's easy to see that the GH3 achieves correct focus more often than the GH2, in fact, about twice as often, according to my frame counting. The GH3 also has a better overall sharpness: It is possible to see the offset printing pattern more easily with the GH3. This could be partially due to the multi aspect sensor feature that the GH3 misses: It means that you get slightly narrower field of view when using the GH3, as compared with the GH2, and hence, more enlargement of the subject.

One of the new features of the GH3 is the 240fps contrast detection autofocus (CDAF) sensor readout. The GH2 only does 120fps, maximum. The smaller print in the GH3 specifications state that the 240fps is only possible when using the newest f/2.8 zoom lenses, the Lumix X 12-35mm f/2.8 and the Lumix X 35mm-100 f/2.8.

And can the camera use the 240fps feature during video recording? Probably not, since the sensor is busy reading the image at 25fps for the video stream anyway. I previously compared the GH2 video AF performance during 25fps and 50fps video, and found that it does better at 50fps, indicating that more frequent image readout is better for the AF performance. As long as the shutter speed is faster than the video rate, there is surplus time between the frames for CDAF readout. Perhaps the GH3 camera can utilize this for better AF performance?

Conclusion

The Panasonic GH3 camera appears to be able to focus better during video recording than the GH2, even at the same frames per second (fps) rate.



9 comments:

  1. can you try the same test with 50fps please ?

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    1. At 50fps, both cameras would perform a bit better in terms of AF, since they sample time image more frequently. I don't plan to do the direct comparisons now.

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  2. Interesting test - thanks.

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  3. I have a basic question regarding still shooting on the GH3. When I shoot a moving subject, the image remains captured in the viewfinder for a moment preventing me from following the subject and shooting again. Is there a way (in the settings perhaps) to prevent this and enable me to shoot something moving several times without losing sight of it?

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    1. It is the "Auto Review" option which does this. By default, it is set to 2 seconds. You can set it to off, to disable this feature. Look in the "Custom Setup" (wrench with C) section, page six, to find this "Auto Review" option.

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  4. Fredrik--Your quick response is much appreciated.

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  5. Which AF settings do you use shooting video? AFF or AFC? Do you use AF Tracking, 23-Area, 1-Area?

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    1. AFF is only an option for AF-Single. I used AF-Continuous, where this is not an option.

      I used the Auto-AF-mode, where the camera decides itself what to look for, and enables face detection if needed. I think it is called "1-area AF in Face Detection". It is a good mode for general use, I find.

      Focus tracking doesn't make so much sense here, since the subject is not really moving (sideways). Only the distance is changing.

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