Panasonic GH1 Video Recording

by Dave Etchells

The big news about the Panasonic GH1, of course, is its video recording capability. More specifically, it's about the fact that the Panasonic GH1 is the first (and so far only) SLR-style camera that can autofocus during live video recording.

For video fans, much of the allure of the Panasonic GH1 is the ability to use a variety of lenses with it, opening a range of creative capabilities. The large size of its sensor relative to those of typical consumer camcorders means that you can achieve shallow depth of field with large-aperture lenses. These things have been possible with digital SLRs for some time now, but the Panasonic GH1 brings this ability in a much smaller package than previously available. Another thing that was unprecedented in the GH1, but that has since appeared in other cameras (the Canon EOS 5D Mark II, with the latest firmware update, and the new Pentax K-7) is the option of full manual exposure control, adjusting both shutter speed (1/30 to 1/4,000 second) and aperture manually.

For consumers, though, the key feature is the Panasonic GH1's live autofocus during recording. Pros and advanced amateurs can "pull focus" (adjust the focus manually) while filming video, but doing it well is very much a learned skill, something very few people ever learn to do really well. Without live AF, consumers for the most part are reduced to only shooting things at a constant distance from the camera - or to putting up with a lot of poorly-focused video. A lot of video-capable SLRs are certainly being sold to consumers these days, and having some video capability is certainly better than none, but for most consumers to make full use of a video camera, it really needs to be able to focus on the fly.

Arguably, this was what Panasonic had in mind from the very beginning of the Micro Four Thirds concept; a true "hybrid" camera that needn't make any excuses for its performance, whether shooting still of video imagery. When I was first briefed on the G-series concept by Panasonic's engineers in Japan, months before the initial announcement, a lot of my discussion with them revolved around the autofocus system. My concern had to do with whether contrast-detect AF could be fast enough to rival the performance of phase-detect AF systems in conventional SLR cameras. While they were certainly concerned about the AF speed for still photos, it turns out that a lot of the decisions made in the G-series AF system were actually driven by the needs of video recording: A critical goal of the G-series design was for the autofocus system to execute a complete AF cycle in 1/60 second -- because that's the basic frame time of an HDTV signal.

The Importance of the Lens

What the engineers were hoping to develop was not just a responsive still-camera AF system, but one that was fast enough to perform autofocus on the fly, while live video was being recorded. This was really a quantum leap beyond anything that had remotely been attempted in an interchangeable-lens hybrid camera prior to that time. To accomplish this, they needed a system and processor fast enough to read out a line of video data, compute the contrast magnitude for it, decide which way to move the lens, accelerate and move the lens and stop it at a precise location again, all within 1/60 second. The emphasis on stopping the lens movement is there because this turned out to be one of the tougher challenges. Accordingly, a good part of the ability of the GH1 to autofocus during video recording depends on the lens you're using with it. Only certain lenses, marked as HD-capable will support live video AF, and the first such is the (excellent) 14-140mm optic that ships with the GH1 body as its kit lens.

We found that we could generally eke out at least some live autofocus from the Panasonic GH1 with other, non-HD lenses, but the kit lens was faster and much more sure-footed than others we tried with it. That said, you can put any lens that you can find an adapter for onto the Panasonic GH1 and shoot with it using manual focus.

There's another lens consideration that's important for video recording: HD-certified lenses for the Micro Four Thirds system have continuously variable aperture settings. That is, the camera can adjust the lens aperture smoothly across the entire range of settings. Most lenses apparently can only vary their aperture setting in fixed steps. Such lenses will produce visible jumps in brightness if the light varies during video recording. (One way around this could be to record in constant-aperture mode when shooting videos with these lenses.)

Panasonic GH1 Video Basics: Speeds and Feeds

The Panasonic GH1 gives you two broad choices of video recording format, and several options for resolution and/or compression level within each of those file types. The table below lists these options.

Panasonic Lumix DMC-GH1 Video Options
AVCHD Format (.MTS files)
Menu Designation	Resolution	Frame Rate	Card Capacity
FHD	1080i 1,920 x 1,080	60i recording* 17 Mbps *(Sensor runs at 24 fps)	2.1 MB/second (15.5 minutes on 2GB card)
SH	720p 1,280 x 720	60p recording 17 Mbps	2.1 MB/second (15.5 minutes on 2GB card)
H	720p 1,280 x 720	60p recording 13 Mbps	1.6 MB/second (20.5 minutes on 2GB card)
L	720p 1,280 x 720	60p recording 9 Mbps	1.1 MB/second (29 minutes on 2GB card)
Motion JPEG Format (.MOV files)
Menu Designation	Resolution	Frame Rate	Card Capacity (very approximate)
HD	1,280 x 720 (16:9 aspect ratio)	30 fps	~~8 MB/second (~4 minutes on 2GB card)
WVGA	848 x 480 (16:9 aspect ratio)	30 fps	~~3.5 MB/second (~9.5 minutes on 2GB card)
VGA	640 x 480 (4:3 aspect ratio)	30 fps	~~2.7 MB/second (~12.5 minutes on 2GB card)
QVGA	320 x 240 (4:3 aspect ratio)	30 fps	~~0.7 MB/second (~50 minutes on 2GB card)

AVCHD vs Motion JPEG: Which to use?

There are significant differences and trade-offs between the two different video formats. The GH1's manual suggests using AVCHD for movies that will be played back on your HDTV via the camera's HDMI connection, and Motion JPEG for playback on a PC. AVCHD uses an H.264 MPEG codec to compress the video stream, which achieves much greater compression than Motion JPEG, but requires more processing power to both compress and decompress. Recent-model computers with a good H.264 codec installed should have no trouble playing back AVCHD video from the Panasonic GH1, but owners of older machines may find their CPUs struggling somewhat, resulting in jerky playback or dropped frames. Editing AVCHD likewise requires a powerful computer (at least by 2009 standards) and good editing software. On the other hand, as suggested by the manual, AVCHD playback from the camera to an HDTV via the camera's HDMI port works just great: HDTVs have very powerful custom processors built into them, designed to handle the rigors of HDTV decoding and display.

By contrast, Motion JPEG (.MOV format) files from the Panasonic GH1 should open and play back just fine on most relatively recent-model computers. On older machines, with slow video cards or slow hard drives, you may still notice occasional hesitations in the playback, but you'll encounter far less than with AVCHD. Likewise, .MOV files are much easier to edit on computers with only average processing power.

The difference in file sizes between AVCHD and Motion JPEG is pretty dramatic, though: With highly-detailed subject matter, 720p resolution AVCHD files will typically be a quarter the size of Motion JPEGs for any given recording length. Also, on the Panasonic GH1, 1,280 x 720 pixels is the maximum resolution available in Motion JPEG, while AVCHD recordings can go as large as 1,920 x 1,080 pixels (full 1080i HD resolution). If playback on a less-powerful computer isn't an issue for you, and you want to record the maximum amount of video in the smallest amount of space, AVCHD will be the way to go. At the highest quality settings (with 17 Mbps data rate), AVCHD video files amount to only about 2.1 megabytes/second, or roughly 128 MB/minute. That's still a lot of data, but is nothing like the file sizes you'd see with Motion JPEG. (Image quality with moving subjects will take a hit, but the lowest-quality AVCHD setting on the GH1 burns memory at a relatively parsimonious 68 MB/minute. At that rate, a 2GB memory card can hold up to 29 minutes of video.)

AVCHD and Motion Artifacts

Obviously, nothing comes for free, so there's got to be a trade-off with AVCHD, right? As it turns out there is, and it can show up big time when you have a lot of motion in the scene you're recording.

There are a couple of different flavors of AVCHD compression, in terms of the amount of data that's recorded. The highest (and apparently somewhat rare) spec results in a data rate of 24 Mbits/second being recorded onto the media, while the more common specification produces 17 Mbits/second. The Panasonic GH1 uses this latter specification. Either spec results in fairly extreme levels of image compression, which means you can encounter some pretty dramatic loss of detail if a lot of the scene is changing from frame to frame. The critical issue is how much of the frame changes between frames, because AVCHD uses a keyframe-based compression. That means that it records full image information (albeit still compressed) only every so often, and in between these "key frames," only frame to frame differences are recorded. It furthermore limits the recording to a particular data rate (the 17 Mbits/second mentioned above), so if there's a lot of change happening between frames, a lot of data can get thrown out. On the other hand, if most of the image is the stationary, with only a small subject in motion, detail remains pretty good all over.

It's important to note that this loss of detail isn't in any way limited to the Panasonic GH1; it's a characteristic of any AVCHD device (although those using the higher 24 Mb/s data rate limit will do somewhat better). The crops below show what we saw in the full-HD video clip above when I panned the camera to follow Charlotte:

AVCHD compression at work in the Panasonic GH1, at 1080i resolution
Here's a 1:1 crop from the start of the video. The camera's not moving, nor are any of the subjects. Not fantastic by digicam standards, but way better than old NTSC video. (In the reduced screen shot of the full-frame image in the first set of sample movies further down this page, notice how this crop represents only a small portion of the overall frame.)	Here, Charlotte is just taking off, and I haven't started panning the camera much yet. As a result, there's still a reasonable amount of detail in Charlotte, but there are also patches where the detail is being suppressed. If you look closely at the grass, you'll also see local areas where detail is a little smooshed. (A technical term. ;-)
Here, the camera and Charlotte are both moving more. Note how much more detail is lost in the grass blades and in Charlotte's coat.	This crop represents pretty much the worst case of what we saw, but it is pretty bad. Soon after this, as I stopped panning the camera so much (Charlotte was running more or less straight away from me), the detail came back fairly quickly. But it was still very noticeable on a big HDTV screen when detail was lost like this.

As it turns out, the subject in the shots above probably represents close to a worst-case for an AVCHD video recorder: There's a tremendous amount of detail in the blades of grass, foliage in the trees, flowers, stonework on the house, etc. The nature of the subject (Charlotte chasing her Frisbee) also required quite a lot of panning to follow the action. Panning is a worst-case scenario, too, because everything in the frame is changing all at once.

The above helps illustrate a couple of things to do to improve image quality in AVCHD videos:

• Subjects with large areas of relatively little detail (flat walls, sky, etc) will leave more video bandwidth available to record detail in parts of the image you're more interested in. (For instance, Charlotte would have been rendered with much more detail if the setting here had been a paved parking lot with a flat concrete wall in the background.)
  
  
• Unless you have to move quickly to track a fast-moving subject, you should always pan slowly when making videos. Most amateur videographers pan their cameras way too fast, making their videos hard to watch. With AVCHD, though, you pay a double penalty for this, as the rapidly-changing image content means loads of detail gets thrown away by the compression.
  
  
  

Best Bet for AVCHD: 720p Recording

The above examples are from a video recorded with the Panasonic GH1 at its "Full HD" setting of 1,920 x 1,080 pixels. In this mode, the sensor runs at 24 fps, but the playback is at 60 frames/second. That's a whale of a lot of data to squeeze into a 17 megabit/second data stream.

It turns out things get a whole lot better if you're willing to drop down to 720p resolution. This is still a pretty big, sharp picture, certainly compared with the 640x480 pixel VGA standard (2x the detail horizontally, 1.5x vertically, a total of 3x as much detail as in a VGA image), but recording at the Panasonic GH1's "SH" setting for AVCHD makes a huge difference in image integrity when dealing with lots of subject motion. You'll still see some loss of detail when panning rapidly, but it's nothing like the fingerpaint-like mush we saw above at 1080i.

The images below are screenshots from a video recorded on a different day than those above, but with the same subject and under generally similar conditions. (Note that, while I didn't notice or record what shutter speed the GH1 was recording at for either of these sets of example images, they were both captured in very bright daylight conditions. The 1080i examples above were shot under full sun, the ones below in slightly hazy sun, but still very bright.) Let's take a look at how 720p holds up:

AVCHD compression at work in the Panasonic GH1, at 720p resolution
Here's a 1:1 crop from the start of the video. As in the prior example, the camera's not moving, nor are any of the subjects. There's not a lot of detail lost to compression artifacts here, although the trade-off is that the spiky leaves of the plants in the background show some jaggies.	Here, Charlotte is just taking off again, and I once again haven't started panning the camera much yet. As a result, there's still a reasonable amount of detail in Charlotte, but there are also patches where the detail is being suppressed. A lot of the detail in the front part of Charlotte's body is blurred, and if you look closely at the grass, you'll still see a few local areas where detail is a little blurred as well. (Look at the shadow area just to the left of her chest.)
In this shot, the camera and Charlotte are both moving a fair bit, but the loss of detail isn't nearly as severe as we saw in the 1080i example above.	This crop is from what was about the worst-looking frame I could find: Charlotte is fairly close to the camera, and was still moving fairly fast, so I was panning pretty quickly to follow here. Note how the detail in both Charlotte and the grass around her is considerably compromised. (And no, this isn't a focus problem, as the full frame shows that neither foreground nor background is sharp, either.)

It's clear that the extreme image compression required by the AVCHD standard can have a significant impact on image quality when confronted with a moving subject or too-rapid panning of the camera. At least in the case of the Panasonic GH1, recording at 720p rather than 1080i can result in much better-looking video. At 720p, you can encounter some jaggies around sharp edges, but in our experience, the overall quality of the video was significantly improved.

Panasonic GH1 Audio Quality

If you have the option, you'll always want to use an external mic of some sort for any serious video recording, regardless of the model of camera or camcorder you're using. A good-quality external mic adds so much to the audio quality, they're generally worth their price if you care much about recorded sound. Sound quality needs to be a serious issue to justify such an addition, though, as an external mic significantly increases the bulk of your overall system, and can easily cost a couple of hundred dollars. At least the Panasonic GH1 gives you the option of using an external mic, via a mini stereo phone jack on the side of its body. Many video-capable DSLRs lack this feature.

You needn't feel too bad, though, if price, bulk, or both limit you to using the Panasonic GH1's built-in microphones. Mounted on the top of the camera, they're more than the usual tiny pinhole in the camera front that you'll find on most competing video-capable still cameras. The pair of little mics can record stereo sound, and in our experience did quite a credible job. Audio was pretty crisp, and stereo localization of sound sources was generally excellent.

They did seem to be somewhat more sensitive to sounds closer to the camera than farther away, but our sense was that they did a good job of reproducing what our ears remembered hearing at the time of recording. There's also a wind cut filter that can be applied via an option on the GH1's Motion Picture menu. We didn't have an opportunity to play with this much, but it offers settings of Off, Low, Standard , and High, and did seem to cut wind noise somewhat. (We didn't use the camera in any very windy conditions, though, so can't speak authoritatively on this feature.)

One important note about audio recording with the Panasonic GH1: The camera's exposure system lets you change things like aperture and/or shutter speed while a recording is in process, but the clicking of the control wheel used to effect such changes is very audible on the audio track. If you want to make such on-the-fly exposure changes, you'll definitely want to either use an external microphone, or overdub the audio track to hide the clicking. (Other aspects of camera operation seem very quiet. If we were careful, we could zoom the lens without affecting the audio, and the focus motor did indeed seem pretty silent. We could hear it working if we listened closely during very quiet passages, but it wasn't at all obtrusive, at least on our sample of the camera/lens combo.)

Panasonic GH1 Live Autofocus Performance

As noted earlier, the Panasonic GH1 requires "HD-rated" lenses for best results with live autofocus during video recording. In the manual, Panasonic notes that subject tracking may be compromised when using non-HD lenses in video recording, and that autofocus likely won't work at all with the camera set to the AVCHD/FHD recording mode. Non-HD lenses may work at lower recording resolutions.

This brings up an issue we noticed with the Panasonic GH1 while recording HD video: The camera clearly had a harder time tracking subject motion when it was recording in AVCHD format, and particularly at the FHD quality setting (1,920 x 1,080i pixel resolution). The GH1's autofocus seemed to have little trouble tracking moving subjects when recording at any resolution in Motion JPEG mode, but its response slowed slightly when we switched to AVCHD mode, at any of the 720p quality settings. When we stepped up to 1080i ("FHD") mode, focus response slowed very noticeably.

We don't have a specific test for focus slew rate, but a little playing around with the camera in the lab showed pretty clear differences in focus response time. In the test, we'd switch back and forth between focusing on an object about 15 feet away in the lab and a computer screen only about 15 inches from the front of the camera. Recording video in Motion JPEG mode, the camera took about a second to make the focus transition and lock sharply on whichever subject we had just switched to. In AVCHD "SH" mode (the highest-quality 720p mode), the lock time increased slightly, to somewhere between 1 - 1.5 seconds. When we switched to FHD mode, though, the focus lag increased to something on the order of 3 - 4 seconds. That'll still be fast enough to handle a majority of likely subjects, but kids or pets running directly toward or away from the camera are likely to get out of focus for at least a couple of seconds.

Another tip about video-mode autofocus with the Panasonic GH1: Because it lacks the direct distance-measuring capability of a phase-detect AF system, the GH1 can have trouble responding to too-rapid changes of subject distance. If the focal distance changes very abruptly (as in a quick shift from a foreground to background object, for instance), the camera can have difficulty figuring out how it should move the lens. It knows the scene is out of focus, but isn't sure whether to focus closer or further away. If the amount of mis-focus is extreme, it may end up "hunting" across the full focusing range of the lens, a very slow procedure. In practice, this wasn't a huge problem, but we found that it paid to be aware of how quick a focus shift we were asking the camera to make: Sometimes, panning just slightly more slowly from foreground to background or vice versa made for a big improvement in focus tracking.

Panasonic GH1 Sample Videos

Click on the links below to view/download our sample clips. You should have no problems playing the Motion JPEG files with most video players, including QuickTime. You may need to install an AVCHD codec or player to view the .MTS files.

(NOTE that these are extremely large files: If you think you'll be inclined to want to refer back to them, please download copies to your local hard drive, to save our bandwidth!)

Panasonic Lumix DMC-GH1 Video Samples
Motion JPEG (.MOV)
1280 x 720, 30fps (48.5 MB)	848 x 480, 30fps (18.4 MB)	640 x 480, 30fps (19.9 MB)	320 x 240, 30fps (10.6 MB)
AVCHD (.MTS)
1920 x 1080, 60i [FHD] (sensor output is 24fps) (25.8 MB)		1280 x 720, 60fps low quality [L] (sensor output is 60fps) (12.2 MB)
1280 x 720, 60fps high quality [SH] (sensor output is 60fps) (20.9 MB) (This is a different scene from that above, shot on a different day. Conditions were very similar, but the sun was rather hazy, so not quite as bright as in the samples above. This gives a good idea of the reduction in motion artifacts when recording at the highest-quality 720p setting vs at 1,080i as in the example above, right.)

Overall, we found the resolution in the Panasonic GH1's videos to be quite good, although we don't have an HD camcorder here to compare them to: We suspect that a true camcorder would manage better sharpness and detail in HD mode, although much would depend on just how the camera compresses its video. Motion JPEG shots from the Panasonic GH1 look fine when compared to other .MOV movies we've shot with digicams in the past. We don't have a standardized test subject to use for these, so any movie image-quality comparisons we make are pretty subjective. That said, though, the GH1's movies seemed pretty crisp and full of detail. As noted above, we found that the limited bit rate of the AVCHD format caused significant motion artifacts when recording a rapidly moving subject, or when panning the camera quickly: Our first choice would be to record at the highest 720p quality setting: The minor jaggies we saw there were more than worth the reduction in compression artifacts.

Panasonic Lumix DMC-GH1 Night Video Samples
Motion JPEG (.MOV)
1280 x 720, 30fps ISO 1,600 (50.0 MB)		640x480, 30fps ISO 1,600 (14.7 MB)		640x480, 30fps ISO 1,600 (20.7 MB)
AVCHD (.MTS)
1920 x 1080, 60i [FHD] (sensor output is 24fps) ISO 1,600 (33.5 MB)			1920 x 1080, 60i [FHD] (sensor output is 24fps) ISO 800 (28.9 MB)

One of the cool things about the Panasonic GH1 (shared with its video-capable SLR competitors) is the ability to shoot at high ISOs for nighttime video recording. I shot some pretty rough, hand-held nighttime videos the first weekend we had the GH1 production sample, when we went to see the new Star Trek movie (pretty cool flick, btw). The examples above show results at both ISO 800 and 1,600, in both Motion JPEG and AVCHD. All in all, not at all bad! -- At least, if you can ignore the jerky camera motion; I really should have used a tripod, but didn't think to bring one along. I was particularly prone to rotating the camera off-axis when I twisted the zoom ring on the lens, something other users may want to be aware of and watch for. Despite my less than stellar camera technique, though, the videos here show that the GH1 can indeed do a very passable job of shooting video after dark.

The AVCHD files here do show the somewhat sluggish AF speed we consistently saw with the Panasonic GH1 operating in that mode. When in AVCHD mode, autofocus remains active during video recording, but can be quite sluggish when compared to its speed for still images or when recording movies in Motion JPEG. You could see some of this in the Charlotte videos above, but it's very evident in the night shots above. (Unfortunately, I didn't know about the significantly improved focus response when recording AVCHD in 720p mode; both AVCHD examples shown above were shot with the FHD setting, producing 1080i video.

Overall, we found we much preferred recording in motion JPEG, despite its much larger file sizes. BUT, if you're planning on watching your movies on an HDTV, the AVCHD format generally looked much better when viewed that way. As noted earlier, the camera and manual both emphasize this, telling you to use AVCHD for viewing images on a TV and Motion JPEG for viewing on a computer.

Bottom line, while we have some issues with the AVCHD recording format in general, we found the Panasonic GH1 to be a highly capable video recording tool!