Panasonic Lumix DMC-GH4 / Sony Alpha 7S Comparative Review
Movie mode cont... (a7S)
The effect of sensor size: Sony a7S
By comparison, the a7S's sensor is significantly larger than the most common movie film formats. This isn't necessarily a problem - most of Zeiss's CINE range of prime and zoom lenses are designed to cover a sensor this large, but it does limit the options in terms of using lenses designed for the smaller (essentially APS-C-sized) Super 35mm format.
The a7S has only a handful of native lenses currently available but has a short-enough flange-back distance to make use of most full-frame DSLR lenses, significantly widening the choices. Stills lenses are not so easily integrated into pro-level rig setups, though, since they lack the standardized gear teeth for focus and aperture control (the gearing rate of the focus and propensity to 'focus breathe' are also drawbacks).
|The short flange-back distance of the a7S allows the use of a broad range of photographic lenses, via adapters. Some cine lenses can also be used but some will require the use of the camera's Super-35mm-like APS-C crop mode.|
The a7S also has a video mode that uses an APS-C crop of the sensor, which broadens the number of lenses it can work with. However, since it's based on a crop, the quality of this output differs from its full-sized footage, since it's around 1.5x oversampled, rather than 2.2x, meaning it's not sensible to mix between the two in a single shoot.
Here, again are the horizontal angles of view offered by a series of focal lengths when mounted on ANSI Super 35mm and on the Sony a7S (if the lenses project a wide-enough image circle).
|Focal Length||Angle of view
(ANSI Super 35mm)
|Sony a7S||Sony a7S
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 Sony offers very little control over AF when in video mode, with no other option than AF-C provided. Generally the camera appears to ignore the specified autofocus point and set the focus where it feels appropriate. Half pressing the shutter button forces the camera to refocus to the chosen focus point.
The a7S offers two different means of engaging focus tracking: Center Lock-on AF, which requires that your subject be in the center of the frame when you first acquire a lock, and Lock-on AF modes based on the standard AF area modes. Only the Center Lock-on system (listed separately in the menus and as a separate Fn menu option) is available in movie mode, meaning you have to find a way of getting your subject into the center of the frame in order to achieve a tracking lock before you shoot.
Repositioned focus example
|1920x1080 24p 50Mbps, XAVC-S, 26 sec, 159.3MB; Click here to download original file|
The video shows that it's not really practical to manually re-position the autofocus on the a7S. Part of the problem is that when you half-press the shutter to force the camera to re-focus, it does so very quickly and with conspicuous flutter before settling on a focus depth. The other problem is that the effort required to move the focus point (something of a weak point for the camera as a whole), ends up introducing shake to the footage.
Tracking focus example
|1920x1080 24p 50Mbps, XAVC-S, 12 sec, 65.4MB; Click here to download original file|
The tracking example works in the sense that the camera was consistently able to keep track of where the subject was. However, the slow refocusing (usually desirable to avoid jarring jumps in focus), means that the subject spends much of the video out-of-focus. So, although the camera did a great job of identifying and following the subject, it's not able to reliably produce usable footage.
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