Movie mode cont... (GH4)
The GH4's most obvious strength is the ability to shoot 4K footage in-camera. This and the greater choice over video codecs makes the GH4 a much more flexible stand-alone camera. It's quite possible to add an external recorder to the a7S but this further widens the already substantial gap in price between the two cameras, as well as having a significant impact on portability. Neither of these are likely to be of concern in many professional settings, of course.
As we'll see later in this review, the GH4's smaller sensor counts against it, in terms of still image quality. Quite simply, the smaller sensor will receive less light at matched exposures (same shutter speed and aperture) and this means more noise. However, the a7S's low-light advantage is less clear-cut when shooting video (where significant changes in shutter speed will change the appearance of movement in your footage). The low-light advantage will only be available when working with shallower depth-of-field than the GH4 can offer (which may be desirable, depending on what you want to achieve). However, if your composition requires a certain depth-of-field, the sensor size advantage is lost as soon as you match the two.
The effect of sensor size: GH4
The GH4's smaller sensor is something of a disadvantage when it comes to absolute stills image quality, but it's not necessarily such a drawback for video shooting. This is aided by the fact that, in addition to the extensive range of native lenses designed for the Micro Four Thirds system, just about any other lens can then be used on the GH4, with an adapter.
The 2X crop factor may be a bit limiting when it comes to using non-native photographic lenses, since anything longer than a 21mm lens ends up as a more-than-normal equivalent. However, it does allows the use of all 35 and Super 35mm cinema lenses via one of the many adapters available, which won't necessarily work on a larger sensor.
Additionally, Metabones offers a series of 0.71x 'Speed Boosters' which work like a back-to-front teleconverter, condensing the light projected into them, reducing the 'crop factor' from the sensor's perspective and increasing the f/number (since the focal length is being effectively shortened but the aperture's diameter remains the same). The company currently makes a Nikon F-to-Micro Four Thirds adapter but Canon EF and Arri PL adapters are also expected.
Unlike stills, where 'full frame' (36 x 24mm) is used as a reference point, many movie shooters use Super 35mm (24.9 x 14mm) as the standard. It's also common to compare lenses in terms of horizontal angle of view, rather than 'crop factors' based on the image diagonal. Here is a list of a series of focal lengths and the horizontal angle-of-view they offer. The first column shows the angle-of-view when mounted on an ANSI Super 35mm film camera, then the angles of view offered by the GH4 in 1080 and 4K modes and then the GH4 in 1080 and 4K modes with a 0.71X Speed Booster attached.
|Focal Length||Angle of view
(ANSI Super 35mm)
|GH4+0.71X Speed Booster (1080)||GH4+0.71X Speed Booster (UHD 4K)|
Sadly, the 0.64x Speed Boosters designed for the Blackmagic Cinema Camera are not considered compatible with the GH4, since there are circumstances in which they can foul the camera's shutter. This is a shame, since the greater focal length reduction would work well with the greater crop of the GH4's 4K mode.
Ultimately, you can see that the GH4 offers a significant crop, relative to Super 35mm (especially in 4K mode), and that this is most likely to be a problem if you're looking to shoot super wide-angle footage. A device such as a Speed Booster can help compensate for this problem - making the GH4 very competitive.
Autofocus in video
Manual focus remains the most effective way of ensuring top-quality video, but we wanted to see how effective the cameras are in terms of autofocus. Can the cameras be trusted to handle focus while the beginner videographer gets used to the other aspects of video shooting?
There are two main ways of using autofocus in video: to set the camera to track a subject throughout a clip or to manually tell the camera when to change to a different focus point. Both cameras are primarily based around contrast detection autofocus - a system that assesses focus as the lens scans across different focus distances. It has the advantage of being highly precise but the downside of not having any inherent awareness of distance. This introduces two major problems: firstly, it can tend to hunt for focus in the wrong direction, going further out-of-focus before re-acquiring the subject, and secondly it can be prone to 'fluttering' back and forth to re-confirm that it is still in optimal focus. Both these effects can ruin a piece of footage, which is why manual focus is usually preferred.
The GH4 augments its contrast detection system with what Panasonic calls 'Depth by Defocus' (DFD), a system that analyses out-of-focus regions and attempts to assess how far out-of-focus they are, based on an understanding of the optical characteristics of the company's lenses. In principle this should help the camera overcome the two main drawbacks of contrast detection autofocus.
The Panasonic's autofocus system changes significantly when shooting in video mode: firstly it ignores the AF mode specified by the AF switch on the back of the camera - the AFS/AFF and AFC positions both behave in the same manner. That behavior is set in the video section of the menu (an option called 'Continuous AF'). When this is engaged the camera will attempt to constantly refocus. Without it, a half-press of the shutter button is required. AF Lock doesn't function in video mode.
The other AF method on the GH4 is to engage Touch AF in the Touch Settings section of the Setup menu. This causes the camera to refocus to the subject that's been touched on the rear LCD. This works regardless of the Continuous AF setting in the menu and attempts to gently pull focus between two points.
Touch Focus example
|1920x1080 24p 200Mbps, MOV, 15 sec, 63.5MB ;Click here to download original file|
Touch focus works really well in this situation - progressing fairly smoothly from one point to the other and back. Clearly it's not as good as a professionally executed manual focus pull, but it's likely to be used if you're a beginner or sole operator trying to get usable footage in a situation where you can't take control of every last setting.
Tracking focus example
The subject for focus lock can be specified anywhere on the screen, so you don't have to force your subject to move (or risk ruining your composition) to get the subject into the center of the frame.
|1920x1080 24p 200Mbps, MOV, 10 sec, 39.7MB ;Click here to download original file|
The tracking focus also works very well - not quite keeping up with the subject throughout its movement but re-acquiring focus without too much fluttering, so the footage remains usable (albeit not by professional standards). However, it is worth noting some slight focus flutter at the end of the video, when the subject is no longer moving.
The only disappointing experience we had was that it took several attempts to get the GH4 to successfully lock onto the target. Just like our experiences with the FZ1000, the focus box with regularly flash red to warn that no lock could be acquired.