Photo answers review
Photo answers rating 
The great thing about traditional DSLRs is that you have a mirror and prism arrangement that enables the camera to use phase detection AF, and you have an optical viewfinder. The downside is that this mirror has to move between each shot and blocks the viewfinder, focus sensors and metering zones during this action. Sony has come up with a solution to stop the mirror moving, but to do this the DSLR has to change. The 16.2MP a55 will introduce this change with a new translucent mirror technology, and will also debut HD video in an Alpha model.
Innovation The a55 is a hybrid design, as it has an electronic viewfinder (EVF) and no prism. It does, however, have a mirror, so it’s a cross between a DSLR and a Compact System Camera (CSC). It looks like a traditional DSLR, so apart from having an EVF instead of an optical viewfinder, what is so different? To start with, no prism/optical viewfinder means it’s not a DSLR, so it becomes a DSLT (T for Translucent). Translucent refers to an innovative (for digital) mirror technology used in this camera where 70% of the light can pass through it to the shutter/sensor chip. This means the mirror can remain in place during image capture. This has never been used on a DSLR before, and the technology enables this Alpha to work in a way never before possible on a digital camera. We have explained the detail of this technology over the page, but in simple terms the a55 is able to shoot faster (up to 10fps), focus more accurately in Live View, plus in HD video capture (introduced for the first time in an Alpha model) you have the option of using the camera’s viewfinder.
Translucent mirror We’ve all seen those police interviews on TV with one-way mirrors, so we know about translucent mirrors. The same concept can be used in a camera. You have phase detection focusing with a mirror, which is far more accurate and provides a quicker focusing performance. It’s therefore better to have a mirror than not. There are limitations to a traditional DSLR design with a flip-up mirror though, such as the time taken to raise and lower the mirror during shots, which actually slows down performance. Plus, with the advent of video capture on a DSLR, you can’t use the viewfinder to ‘film’ so are limited to Live View only. Also, when the mirror is raised the camera can’t actually meter or focus, so it has to do it between actuations and that requires serious processing power at a high frame rate. Having an electronic viewfinder but using a translucent mirror where 70% of light can pass through is better for fast and more accurate focusing. It allows the camera to continue altering focus and check exposure while capturing images, and also allows the viewfinder to be used for video capture. The translucent mirror is, therefore, an important development for a digital camera, and the a55 is set to start a potential revolution.
Build and features This is a well-made camera with a design that looks a lot better and is 23% smaller than the a550. There are plenty of impressive features like the previously mentioned HD video capture which, with the translucent mirror, benefits from Continuous Phase Detection AF. The camera design sees the introduction of a 3in TruBlack LCD that pivots from the base of the camera (rather than the side as on most other designs). We have already mentioned the fact this camera has an EVF, which Sony has named a Tru-Finder on the basis that it gives 100% field-of-view and provides Live View. The quality of this mini screen is superb, and certainly one of the best we’ve seen. This Tru-Finder EVF can also be active during movie capture, giving you traditional camera handling, but for video. Below the EVF is a sensor that switches between the viewfinder and LCD as you raise or lower your eye to the camera – very neat. Creative tools include Sweep Panorama (another first for an Alpha camera), and this also includes 3D capture to be played back on a Sony Bravia 3DTV. For focusing, there’s a new array of focus points, featuring 15 selectable autofocus points with three cross-type sensors. Metering is achieved through a quite phenomenal 1200-zone evaluative system that measures to a very impressive accuracy, and there’s built-in image stabilisation thanks to the Sony SteadyShot INSIDE technology. Auto HDR is a great feature where the camera takes three images at different exposures and then, using edge definition technology, ensures the images line up, making it easy to shoot HDR handheld. Traditional modes like D-Range Optimiser, Face Detection, Smile Detection, and Auto+ (scene recognition) are also included, along with a new GPS tagging option so you can pinpoint exactly where you took your shots. There’s a very sensitive tilt and level gauge to keep you on the straight and level, and built-in dust protection too. The super-compact battery is also very welcome and you have a choice of using either the SD format or the Sony MS Duo memory cards.
Performance The 10fps shooting speed is reserved for a dedicated and slightly limiting mode on the control dial, so if you want to use PASM modes, you'll get 6fps instead. There are limitations with 10fps or 6fps drive speeds in Live View as the camera cannot keep the screen updated, so it merely displays the last captured file. This means you are not quite sure what you are capturing, so it’s best to use the viewfinder (as we suspect most users would prefer in any case). The buffer can also take a long time to empty down if you want to view the files quickly. Other than that, the performance is most impressive. Rather unsurprisingly with so many metering zones (1200), the camera rarely gets the exposure wrong. It also delivers a good tonal range and with the kitted lens the pictures demonstrated accurate contrast and detail. The focusing was also a lot better than any previous Sony model we have tested, and despite not getting images sharp all the time in continuous AF, the hit rate was high.
Final verdict This camera is a marked improvement over the previous models and leads the whole market in a new direction. The mirror technology gives a very rapid performance and impressive AF, while the HD video is the best we have used in this style of product. We especially like the way you can use the excellent EVF for video.
Key spec Street price (body only) £670
Effective resolution 16.7MP
Sensor type 23.5x15.6mm CMOS
Crop factor 1.5x
Lens mount Sony A
Exposure modes PASM, creative modes, HD video
Metering modes 1200-zone evaluative
ISO 100-12,800 (25,600 expanded)
Shooting speed 10fps
Card type MS PRO Duo/HG Duo or SD/SDHC/SDXC
LCD monitor size 3in
Live View Yes
Autofocus 15-point (Three cross-type)
Integral cleaning Yes
Built-in stabilisation SteadyShot INSIDE
Weight/size (wxhxd) 441g/124.4x92x84.7mm