Sunday, 28 October 2012

Continuous AF during video with the GH2

The Micro Four Thirds mirrorless cameras use a totally different autofocus method than traditional SLR and DSLR cameras. The M4/3 cameras use a technique called Contrast Detection Autofocus (CDAF), whereas SLR systems typically use Phase Detection Autofocus (PDAF), also called Phase Difference Autofocus.

CDAF relies on jogging the autofocus back and forth to find the setting with the most contrast, in which case the camera assumes the image is in focus. For still images, this works very well. My study shows that both the GH1 and GH2 feature very fast autofocus, especially with the kit zoom lenses. I don't think that SLR cameras can do this much faster, at least not faster in a noticeable way.

The GH2 can sample the contrast 120 times per second (120fps), which allows for a faster autofocus, given that the lens is also capable on reacting fast enough. The GH1 could only sample at 60fps.

The Panasonic GH3, due to be available late 2012, further improves upon this by allowing for 240fps CDAF readout. However, Panasonic says that at the moment, only one lens supports this, the Lumix X 12-35mm f/2.8, and only after firmware update, which was announced today, coincidentally.

Continuous autofocus during video recording, though, is one area where the Micro Four Thirds cameras currently do not perform very well.

One thing to keep in mind, is that when taking still images, the sensor is free to be used for AF before you snap the image. When recording video, though, the sensor is busy scanning your video stream, and cannot be used for high speed CDAF sampling in the same way.

The GH2 can record videos at three different rates: 24fps, 25fps and 50fps (24fps, 30fps and 60fps for American versions). The 24fps, 25fps and 30fps modes are popular for their progressive image stream, and high bit rate. However, keep in mind that in these modes, the camera can only sample the contrast at a low rate, hence, I guess that autofocus during video will also be slower.

The interlaced 1080i mode, 50fps for the European version and 60fps for the American version, can scan the image scene twice as fast, and hence one can guess that the AF is also faster.

The test

I have tried to test this with a controlled case. I rigged up a picture which moves back and forth (using a LEGO Technic mechanism). This is used to test the autofocus performance in both the 50i and 25p modes of the GH2, using three different lenses, the Lumix X 45-175mm f/4-5.6, Lumix G HD 14-140mm f/4-5.8 and Olympus 45mm f/1.8. All of these lenses are marketed as video optimized.

Here is a video showing the test setup, and the outcomes of the tests:



Analysis

By looking at the videos, I think it is quite clear that the focus is kept better when recording at 50fps, compared with 25fps. To be more sure, I looked through the first full cycle when using the Lumix X 45-175mm f/4-5.6. The cycle from the rear position, moving forwards and then backwards again to the same position takes 140 frames, or 5.6 seconds.

At 50fps, I counted 71 frames in focus (51%), while at 25fps, the number of frames in focus was 59 (42%). This is hardly a scientific analysis, but I think it confirms that my theoretical guess also holds in practice: When recording at a higher frames per second rate, the continuous autofocus works better.

Conclusion

When the continuous autofocus performance is important, you could consider video recording at a higher frames per second rate. This gives the camera a better chance of keep up the focus, given that there is sufficient light.

On the other hand, the GH2 camera can only provide the interlaced mode at 50fps/60fps at 1080 lines resolution. The interlaced mode probably gives slightly less perceived sharpness anyway.

Future cameras are expected to provide the progressive mode at 1080 lines resolution (1080p) at 50fps/60fps. With this capability, I would certainly recommend using 1080p at 50fps/60fps when the autofocus performance is crucial, and when there is sufficient light.

Technical improvement potential

One could imagine improvement potential within the technical limitations of the autofocus system. As I have demonstrated in this article, the frames per second rating affects the autofocus effectiveness. As the sensor is used to record the video stream, it cannot at the same time be used for autofocus at a faster rate.

However, it is quite often that you don't use a 360° shutter. A 360° shutter means that the shutter stays open all the time. Hence, at 25fps video recording, the shutter speed is 1/25s. At this fps and shutter speed combination, the sensor is always busy reading the video stream.

However, what if you use a 180° shutter? At 25fps, that would correspond to 1/50s shutter speed. At this rate, there is a 1/50s gap between each frame which could be used for reading out CDAF information, see the illustration below:


Using this method, there is room for one exposure, and two additional CDAF readouts per frame when using a 180° shutter. Perhaps the GH2 uses this technique today, I don't know. These technical details tend to be secrets.

Alternative techniques for AF during video

The Micro Four Thirds cameras so far only use CDAF for autofocus, both for still images, and during video. There are some other techniques out there as well.

The Sony Single-Lens Translucent (SLT) cameras use a permanent (non-flapping) mirror which is semi-transparent. This mirror allows for using traditional SLR style PDAF sensors, meaning that the camera can operate the focus very quickly also during video recording.

This has an additional advantage over CDAF: There is no need to jog the focus back and forth to find the maximum contrast, as the Micro Four Thirds cameras do. Hence, you will see that the cameras do not "overshoot" the focus. When needed, the focus changes to the correct distance at the first attempt, given that there is sufficient light.

The disadvantage of the SLT system is the requirement for a traditional mirror box, making the cameras larger and more complicated, as well as requiring the lenses to be designed for a longer register distance, resulting in larger and heavier lenses.

The Nikon 1 system has an alternative approach. It uses an imaging sensor where some of the photosites are PDAF sensors. This allows for a hybrid CDAF and PDAF system: The camera can choose which system to use depending on the situation. In theory, this is the best of both worlds, as it can keep the register distance short, while allowing for faster autofocus during video recording. I have not tried the Nikon 1 myself, though, so I don't know the real world merits of the system.

According to interviews with designers and engineers, there are no plans to change the autofocus system of the Micro Four Thirds cameras. In theory, it should be possible to improve the CDAF system with better and faster image processing. Hence, we can expect the continuous autofocus to gradually improve with newer cameras.

2 comments:

  1. Thank you for your excellent test!

    When first announced the Nikon 1 showed a lot of promise for video recording but did not achieve that promise due to low measured video resolution (line skipping?) and slow lenses.

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    1. That is quite interesting to hear. I think the Nikon 1 system has some real potential. So far, however, I think the camera bodies have been a tad large, given the sensor size. Also, the ergonomics do not appear to be very good. But I like the direction the new Nikon 1 V2 takes.

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