The Sony Alpha SLT-A55 may look like just another addition to the Alpha SLR range, but inside there are big changes afoot. Instead of using a flip-up mirror to direct light either to the optical viewfinder or the sensor, it uses a fixed, translucent mirror to split incoming light to both the CMOS sensor and a 15-point autofocus array.
The basic premise isn’t new – it’s the same principle that allows an SLR’s viewfinder and autofocus system to operate simultaneously. However, sending light to both the sensor and the autofocus has numerous repercussions. Together they make the A55 – along with its more affordable sibling, the A33 – quite unlike any other camera currently available.
Performance
Simultaneously active autofocus and imaging sensors give a big boost to continuous performance. Sony capitalises on this with a dedicated speed-priority burst mode on the mode dial, which uses fast shutter speeds to achieve 10fps shooting with autofocus – a feat previously unimaginable this side of £3,000. Exposure control is limited in this mode, but a 5.8fps continuous drive mode is available in other shooting modes.
The large buffer kept the 10fps speed going for 24 JPEGs or 20 RAW images in our tests. However, even when using a fast SDHC card, performance took nosedive once the buffer was full, slowing to 1.7fps for JPEGs and 0.5fps for RAW images. At ISO 1600, it slowed to just 0.8fps for JPEGs, presumably because the noise-reduction processing had to work harder. It also took around 40 seconds to flush the buffer, ready for another burst of shooting. It’s clear that the camera’s processor is the bottleneck here, rather than the memory card – it’s a shame Sony didn’t see fit to use a faster one.
The A55 differs from true SLR cameras in its use of an electronic rather than optical viewfinder. We’ve seen this before in Panasonic’s G-series cameras as well as on countless ultra-zoom models. As with the Panasonic G2, this one’s 1,440k resolution matches consumer SLRs’ optical viewfinders for detail.
Sadly, it doesn’t perform so well in practice. When Image Review is enabled to examine shots just after they’ve been taken, the screen goes blank for about a second after capture until the shot appears. That slow processor is to blame once again, and it isn’t much fun for anyone who’s used to an optical viewfinder. Image Review also makes the camera less responsive – we measured 1.3 seconds from shot to shot in the Single drive mode. Disabling Image Review reduced this to 0.5 seconds, but we never got over the frustration of having to reach for the playback button each time we wanted to review a shot.
Video
A crucial advantage of the A55’s translucent mirror is the ability to use SLR-style phase detect autofocus during video capture – a new feature for the Alpha range. Various other SLRs can record video but so far only the pricey Panasonic GH1 has delivered continuous autofocus that’s responsive enough for discerning use. The Sony Alpha SLT-A55 is just as quick at focusing while recording video as it is when composing photos, and with its 15-point autofocus sensor, it’s very quick indeed.