Sony alpha 55

The Sony Alpha A55 is among the first of a new breed of camera. Dubbed an 'SLT' (Single Lens Translucent), the A55 may appear to be a DSLR on the outside, but its internal construction is quite different.

The mirror inside is translucent, which means light can permanently pass to the sensor while reflected light is always fed to the autofocus sensor.

As technological as this may sound, all you need to know is that this allows for true continuous autofocus that's perfect for tracking moving subjects during movie recording.

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Auto focus

The A55's continuous autofocus works like a dream – indeed it's the best continuous AF system for movie of any stills camera, although it's not without its issues.

Clever though it may be to quickly slide focus between one subject and another, there's little choice of what the camera will focus on, because the camera always focuses on the most central point. Should someone walk through your shot, for example, the camera will quickly adjust to this new subject, even if that's not your intention.

What the A55 lacks is the ability to easily toggle the focus on or off, or manipulate the focus sensitivity. It is possible to pop many lenses into manual focus using their direct AF/MF switches, but this isn't a practical way of 'pausing' the focus.

Exposure

The Sony Alpha A55's movie shooting mode doesn't venture outside of the automated Programme mode either, which means beyond the ability to control exposure compensation and exposure lock (AEL) during capture, there's little else at your disposal.

LCD

Design-wise the Sony Alpha A55 comes equipped with an LCD screen that's mounted on a vari-angle bracket. This means the screen can be moved away from the camera and rotated through a variety of angles. This is most useful for waist-level shooting for lower-angle shots.

Performance

With Sony, as with Panasonic cameras, the choice to use AVCHD as its primary movie capture format has a series of benefits and downsides.

On the one hand, the H.264 compression is very good, because it maintains high quality while rendering small file sizes. The 17MB/s data rate is high for a camera of this type, and a minute of footage equates to around 135MB.

On the other hand, the native MTS files captured aren't useable in most computer programs or editing suites (VideoLAN - VLC can read them, but not always smoothly) and this means files need to be decoded into a different format, such as the larger MOV file type, using (free) software such as Windows Movie Maker or Apple's iMovie.

The 'Full HD' branding of the Sony Alpha A55 comes with a slight pinch of salt: the sensor captures files at 25fps in an interlaced format, ie odd lines are captured on one pass of the sensor, and even lines are captured on the next pass. It can be hard for the human eye to recognise this as a problem – until, that is, fast moving subjects show signs of 'tearing', which is like seeing one frame in a position and the next frame slightly offset from the other half.

Downgrade to Sony's 'AVC' capture and you'll get progressive capture, but at a 1440 x 1080 resolution with a far lower data rate that's then upscaled to Full HD. This secondary option produces MP4 files direct from camera that are more immediately useable, but the quality isn't as top-notch.

Audio

Audio is handled by the camera's built-in microphone, although this will pick up surrounding sounds such as the lens's autofocus. However, flip open a panel on the left side of the camera and a 3.5mm microphone jack means you can record using an external mic, with improved results.

Overall, the Sony Alpha A55 is up there with the best when it comes to point-and-shoot capture and continuous autofocus. It's not 100% reliable, however, because using the sensor's SteadyShot image stabilisation system will cause overheating (resulting in cut out) after around nine minutes, depending on the ambient temperature. It's not an exclusive Sony issue, but it's one to be aware of.

Key video specifications

Approx price: £600 with 18-55mm kit lens
Sensor: APS-C-sized sensor (1.5x magnification)
Maximum resolution: 1080i (1920 x 1080px)
Frame rate: 25fps
Compression and file format: H.264 for AVCHD video and AAC audio compression, AVCHD format requires decoding; 1440x1080 Motion JPEG format playable straight from camera as MP4 files
Exposure mode: Programme mode with exposure compensation, AEL, ISO control and (prior to recording only) aperture adjustment
Connectivity: HDMI-C out port with Bravia Sync, 3.5mm audio jack for external microphones