Panasonic DMC-GH2 Review
Shooting Video with the GH2
|This video showcases the GH2's abilities in a range of different environments and lighting conditions. It also contains a demonstration of the GH2's ETC mode, which increases effective focal length without the need for an external lens adapter.|
The GH1 was one of the first large-format interchangeable lens cameras to offer a 'serious' movie capability and it was hugely competent, both in terms of its still and video capabilities. However, quite a few filmmakers were disappointed by the GH1's video image quality in some situations.
The main problem with the GH1 was muddy, smudged detail which - for some shooting - was a showstopper. It was clear to filmmakers that the low bit-rate of the GH1's AVCHD codec was a limiting factor to image quality and it wasn't long before an unofficial firmware hack (dubbed the GH13) was developed which allowed the bit rate to be increased. The GH1's unusual 1080p 24fps in a 1080i 60fps wrapper footage didn't make Panasonic many friends either. The resulting video stream needed a so-called pull-down conversion to become a proper 24p stream. The 'GH13' hack fixed this problem too.
The other major shortcoming of the GH1 was its relatively poor video image quality at high ISO settings, compared to competitors like the Canon EOS 5D Mark II. This is a problem that cannot be fixed by a firmware hack. Fortunately, Panasonic has listened to the complaints of GH1 users and with the GH2, it has addressed most of the urgent issues. The GH2's bit rate is better than the GH1's, but of even more importance is an improved AVCHD encoder that also incorporates "B-Frames". B (Bi-predictive picture) frame compression saves space by using differences between the current frame, and those which follow and precede it, to assemble its content.
Say a piece of footage consists of a moving object against a motionless (from the camera's point of view) background. By looking backwards and forwards from each frame, the encoder can figure this out, and save space by not storing the pixels from those areas of the scene that do not change.
In other changes, the GH2 can now directly record 1080p, 24fps video, and Panasonic claims that low light performance has been improved as well. However, filmmakers don't have the full run of the GH2's ISO range, and video can only be captured up to ISO 3200.
Sensor SizeThe sensors in video cameras are typically extremely small. This helps to keep the camera bodies (and lenses) compact, but it restricts depth of field control, and also limits dynamic range. The attraction of DSLRs and mirrorless cameras like the GH2 for filmmakers is the larger sensor compared to nearly all other professional camcorders. These larger sensors allow filmmakers to take much more control over depth of field, and also deliver better image quality at high ISO settings, and greater dynamic range.
1920 x 1080, 20.6MB. Click here to download original .MTS file
|This clip, shot wide open with the 14-140mm zoom, demonstrates the sort of isolating effect that you can achieve in video footage shot on a large sensor. These two toys were positioned an equal distance apart from one another, with the farthest roughly 1 meter from the camera. Focus was automatic, and triggered by touch.|
The GH2's frame size is slightly smaller than 35mm cine film, but much larger than the sensors in most pro camcorders. So-called 'full frame' sensors (such as that used in the Canon 5D Mark II) are larger again. Filmmakers love the sort of video that full-frame DSLRs can produce but the extremely shallow depth of field can be a real challenge when it comes to critical focusing. Overall, we consider Micro Four Thirds sensors to be a good practical compromise.
What Lenses for Video?
Today the GH2 can be used via adapters with almost all SLR and cine lenses on the market (most of them limited of course to manual focus). There are many good prime lenses to choose from - anything from M42 to Nikon F mount. Pro filmmakers (mainly, in our experience, those that work with the Panasonic AF100) often use Olympus 4/3 zooms, like the excellent Olympus 14-35mm f/2.0 ED SWD and Olympus 35-100mm f/2.0 ED. These lenses have an excellent reputation but they are bulky, and costly. Here's a list of the lenses that we use most with both the AF100 and GH2:
- Panasonic 14-140mm f/4-5.8 (with OIS, 28-280mm equivalent): A great outdoors zoom. The optics are decent but the maximum aperture at any given focal length is relatively small, which makes this lens a good choice for video in daylight.
- Panasonic 100-300mm f/4.0-5.6 (with OIS, 200-600mm equivalent): This lens packs an amazing range and high quality optics into a small package, which makes it a good choice for wildlife.
- Olympus 4/3 (via adapter) 14-54mm f/2.8-3.5 (no OIS, 28-108mm equivalent): This is the lens we use for indoor videos because it is reasonable fast and has good optics. The Olympus 4/3 12-60mm is faster and a little sharper but it is bigger, heavier and more expensive.
Of course, all of these lenses can be used for stills as well as video. Typically we tend to use an AF100 (for video only) and GH2 (stills and video) with one set of lenses.
Exposure and Metering
Exposure in video is different from stills, but the essentials are the same. It is important to get the exposure as close to correct as possible, in-camera. It is possible to make minor adjustments to brightness post-capture, but just like JPEG images, video files don't stand up well to drastic exposure adjustments and there is no such thing as 'highlight recovery' when working with a video file.
|The GH2 allows monitoring the exposure via a live histogram and blinking highlight warnings. The highlight warnings are very important and helpful indicators, (but flash uncomfortably quickly in our opinion). While these tools are useful, they are primitive compared to the configurable Zebras and Wave Monitors used in professional, dedicated video cameras like the Panasonic AF100.|
As far as exposure control is concerned, with still photos, photographers are used to balancing aperture, shutter speed and ISO to get the optimal exposure. With video you can do the same but the requirements are slightly different. For video you generally want to keep the exposure time constant in relation to the frame rate. So if the frame rate is 24 fps you would ideally use a shutter speed of 1/48th (this is known as a 180 degree shutter - in reality actually 1/50th because the camera cannot set exactly 1/48th). Why would you not want to use - say - 1/250th of a second if the light is bright enough? Because the resulting image would show too little motion blur.
Although it might seem anathema to still photographers, motion blur is important when shooting certain types of video. In still photography we are used to using high shutter speeds to capture motion, but somewhat counter-intuitively, when it comes to capturing motion in video this isn't a good idea. In general, a degree of motion blur in footage of moving subjects is more attractive. The first of these videos was shot in manual movie mode at a shutter speed of 1/25sec (the slowest shutter speed possible in video mode with the GH2, and slower than we'd advise for most purposes), and the second at 1/250sec. The difference is clear. We much prefer the first - the motion blur makes the footage appear smoother and more natural.
1/25sec Shutter Speed
1920 x 1080, 11.23MB. Click here to download original .MTS file
1/250sec Shutter Speed
1920 x 1080, 11.53MB. Click here to download original .MTS file
It goes without saying that keeping the shutter speed down is a challenge outdoors at bright light. We could stop the lens right down but that would eliminate the shallow depth of field that we like so much. The only way to get the exposure time down whilst allowing us to use wider apertures is the use of neutral density (ND) filters. Most professional video cameras, including Panasonic's AF100, have them built-in but at time of writing, no DSLR or interchangeable lens camera offers this functionality. Therefore, the only option is screw-in or drop-in ND filters.
Whatever type you use, external ND filters are a hassle. There is a more elegant solution though - Variable ND filters, which use two opposite oriented polarizing filters. Variable ND filters allow you to dial in different ND filter strengths without swapping or adding additional filters to your lens. The downsides are increased risk of vignetting when shooting at wide angles, and the fact that like all polarizing filters, they change strength when your direction relative to the sun changes (i.e. if you pan the camera).
Using Extra Tele Conversion (ETC)New in the GH2 is ETC mode. ETC stands for Extra Tele Conversion, and is designed to give you additional telephoto 'reach' for video via using a crop of the sensor without losing quality. How can this be? The effective resolution of 1080 HD video is roughly 2MP. Normally, the full output resolution of the sensor is used to capture video but in ETC mode only the center 2MP is used, creating a teleconverter effect, equivalent to 2.6x. To put this in perspective, it means that at the long end of the 14-140mm zoom (equivalent to 280mm), with ETC activated you can shoot at an focal length equivalent to 728mm. The image below shows the effect of activating ETC, and as you can see, it gives a real boost to telephoto performance.
In our experience of using the ETC function, image quality is very good at lower ISO values. Be sure to lock the camera on your tripod though because at extreme focal lengths the slightest breeze can create enough camera movement to ruin a shot. Also, be aware that when using ETC, as with all long lens shooting, atmospheric haze in the distance can reduce critical image quality.
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