Video AF comparison, Lumix 25mm f/1.4 vs 20mm f/1.7

The Lumix Leica DG 25mm f/1.4 and Lumix G 20mm f/1.7 pancake are quite similar. Both are fast normal lenses. Still, they are different:



On both lenses, I am using 46mm to 37mm step down rings as lens hoods. If you go down the same route, you will also need a 37mm front lens cap.

Beyond the size difference, the 25mm lens features internal focusing, while the 20mm lens has an old style focus mechanism, where the whole lens array moves back and forth. The internal focus achieves faster focus, and makes less noise. The Lumix 20mm lens on the other hand is known to focus slowly, and for making more noise.

In this article, I aim to see if the difference in focus mechanisms make the Lumix Leica DG 25mm f/1.4 better suited for continuous autofocus during video.

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.

GH1 autofocus speed comparison

I have made some tests of the autofocus speed of the Panasonic Lumix GH1 camera with various lenses. The camera focused from infinity (the default position of the lens when powering down) to near the minimum focus distance, and I used a LEGO figure as the subject.

The test was done in indoor lightning, about EV6. The focus time is measured as the time from my finger presses the shutter button until the green focus confirmation light comes up in the display. The picture is taken immediately after focus is achieved, within one tenth of a second.

You will hear the shutter operating twice, since the camera was in multi exposure mode. I did confirm that all the images were indeed in focus, as is expected with a contrast detection autofocus system (CDAF).

Summary

Before going into the details, here is a quick summary

Lumix G 20mm	1.23 seconds
Lumix G HD 14-140 @ 18mm	0.53 seconds
Lumix G HD 14-140 @ 50mm	0.40 seconds
Lumix G HD 14-140 @ 140mm	1.63 seconds
Lumix G 45-200 @ 45mm	0.33 seconds
Lumix G 45-200 @ 100mm	0.36 seconds
Lumix G 45-200 @ 200mm	0.87 seconds
Olympus 4/3 9-18 @ 9mm	2.90 seconds
Olympus 4/3 9-18 @ 18mm	1.50 seconds

What is a bit surprising here, is that the 45-200mm lens is quicker than the HD 14-140mm. The latter is marketed as a very quick focusing lens, optimized for video, hence the HD designation. However, the 45-200mm lens has an advantage, since it's close focusing distance is 100cm, twice that of the HD 14-140mm. So when focusing from infinity to the minimum focusing distance, the HD 14-140mm has a longer way to travel.

Lens: Panasonic Lumix G 20mm f/1.7

Autofocus from infinity to 23cm: 1.23 seconds.
The minimum focus distance of the lens is 20cm.

Lens: Panasonic Lumix G 14-140mm HD f/4-5.8

Focal length 18mm (36mm in 35mm camera equivalent), f/4.3:

Autofocus from infinity to 53cm: 0.53 seconds.
The minimum focus distance of the lens is 50cm.

Focal length 50mm (100mm in 35mm camera equivalent), f/5.6:

Autofocus from infinity to 53cm: 0.40 seconds.
The minimum focus distance of the lens is 50cm.

Focal length 140mm (280mm in 35mm camera equivalent), f/5.8:

Autofocus from infinity to 53cm: 1.63 seconds.
The minimum focus distance of the lens is 50cm.

The outcome for f=140mm deserves some more comments. As you can see from the video, the focus is hunting a bit before settling. I tried to redo this experiment several times, and found that the outcomes were very consistent. My speculation is that I have been operating close to the minimum focus distance, and that perhaps this distance is slightly longer in the tele setting. Indeed, moving the subject a bit further away from the camera gave focus speed consistent with 18mm and 50mm focal lengths.

I have also tested this with the firmware versions v1.2 and v1.3, and concluded that the issue has been fixed. The lightning was comparable, and even though I moved the figure around close to the minimum focus distance, I was not able to reproduce the focus hunting. Rather, the focus speed has improved a lot with the newer firmware, it seems.

Lens: Panasonic Lumix G 45-200mm f/4-5.6

Focal length 45mm (90mm in 35mm camera equivalent), f/4.0:

Autofocus from infinity to 1m: 0.33 seconds.


Focal length 100mm (200mm in 35mm camera equivalent), f/4.6:

Autofocus from infinity to 1m: 0.36 seconds.

Focal length 200mm (400mm in 35mm camera equivalent), f/5.6:

Autofocus from infinity to 1m: 0.87 seconds.

Lens: Olympus Zuiko Digital ED 9-18mm f/4-5.6

Note that this is not a Micro Four Thirds standard lens, but rather a lens for the Four Thirds DSLR system. To mount this lens on a Micro Four Thirds camera, you will need and adapter. I used with the adapter Panasonic DMW-MA1, but the Olympus MMF-1 adapter is functionally the same, and would have done the same job.

Not all Four Thirds lenses can autofocus on Panasonic Micro Four Thirds bodies, like the GH1. Here is a list.

Focal length 9mm (18mm in 35mm camera equivalent), f/4:

Autofocus from infinity to 25cm: 2.90 seconds.
The minimum focus distance of the lens is 25cm.

Focal length 18mm (36mm in 35mm camera equivalent), f/5.6:

Autofocus from infinity to 25cm: 1.50 seconds.
The minimum focus distance of the lens is 25cm.

Conclusions

The autofocus speed of the Lumix G HD 14-140mm lens is the fastest in this comparison, which is as expected. The unexpected result in this context was the significantly slower autofocus speed at full tele, 140mm, however there is reason to believe that this was related to operating close to the minimum focus distance, as discussed above.

Just as with the superzoom above, the Lumix G 45-200mm features very impressive autofocus speed. The exception is at full tele, however, the speed at 200mm is still very good. The autofocus is virtually inaudible.

The Lumix G 20mm lens does indeed focus slower than the HD lens, and also somewhat more audibly.

Using autofocus with the Olympus 9-18mm Four Thirds lens is possible, but pretty slow. Especially at the wide angle setting. Focusing with this lens is also quite noisy. Taking pictures of moving subjects, e.g., children, with this lens could pose some difficulty with autofocus. In this case, it could be wise to prefocus, and set the camera to manual focus (MF) while composing the image. That way, you can take the picture nearly instantly when pressing the shutter, rather than having to wait some seconds for the autofocus to settle.

Lumix G 20mm	1.23 seconds
Lumix G HD 14-140 @ 18mm	0.53 seconds
Lumix G HD 14-140 @ 50mm	0.40 seconds
Lumix G HD 14-140 @ 140mm	1.63 seconds
Lumix G 45-200 @ 45mm	0.33 seconds
Lumix G 45-200 @ 100mm	0.36 seconds
Lumix G 45-200 @ 200mm	0.87 seconds
Olympus 4/3 9-18 @ 9mm	2.90 seconds
Olympus 4/3 9-18 @ 18mm	1.50 seconds

Mostly, you will not focus down to near the minimum focus limit of the lens, and so autofocus will usually be faster than these examples. The Olympus 9-18mm lens is a bit of an exception to this, however, as even focusing on a distant subject takes virtually as long time as focusing close.

Rumors say that future Panasonic models, like Lumix G2 and Lumix G10, will focus faster with Four Thirds lenses on an adapter.