The mirror in the A55 is semi-transparent. According to Sony, it allows approximately 70% of the light that strikes it straight through to the 16.2MP CMOS imaging sensor. The remaining proportion is reflected upwards to a large 15-point phase detection AF array. This equates to a roughly 1/2 EV reduction in light transmission for live view/image capture but means the AF sensor is always receiving light.
This isn't an entirely new idea - fixed, semi-transparent mirrors have been built into SLRs before, but the system in the A33 and A55 is a major new development. What makes the A33 and A55 unique is that they can maintain phase-detection AF at all times, even during an exposure.
They manage this by removing the optical viewfinder entirely and only having to direct light to the phase-detection AF sensor array. For packaging reasons this has been moved from its conventional position at the base of the camera's mirror box, to its roof. Roughly 30% of the light that strikes the main mirror is reflected upwards, onto the AF sensor array. Meanwhile, in the absense of an optical finder, the main imaging sensor provides a full-time live view image via an electronic viewfinder in its place.
There is still a conventional mechanical shutter so operation isn't silent, however.
The optical system of the A33 and A55 is unique, and represents a significant step forwards from conventional SLR design. Both cameras are built around semi-transparent mirrors, which allow light to pass through them, for the purposes of live view and exposure, but also reflect a portion of the light onto a new phase-detection AF array. Here's a detailed look at this and other new technology used in the SLT A33 and A55.
*From this camera position, the mirror appears to be fully reflective. In fact, only around 30% of the light which strikes it is reflected (onto the A55's phase-detection AF sensor) and approximately 70% of the light which strikes it is transmitted...
*...a change of camera position reveals the mirror as a 'window' through which around 70% of the light which enters the camera passes - falling on the imaging sensor.
This view shows the semi-transparent mirror in its normal (down) position and manually flipped up for sensor cleaning. In the down position, you can see the 15-point AF array reflected from its position in the roof of the mirror box.
A newly developed 15-point AF array is recessed inside the roof of the A55's mirror box. Three of the 15 AF points are of the cross type (shown here in red) for more accurate focus with large-aperture lenses.
The A55's EVF is a Sony-manufactured, 1.44 million dot field-sequential display which provides a 100% field of view. This image shows a simulated view through the finder when shooting in the 16:9 aspect ratio.
In a first for Sony's Alpha series the 912k dot TruBlack LCD on the rear of the A55 is fully articulated.
The A55 features Sony's established Super SteadyShot INSIDE image stabilization system to reduce the effect of camera shake. Sony claims a benefit of between 2.5 and 4 stops in useable shutter speed.
The A55 has a built-in GPS receiver, which can automatically tag photographs with location data. This data can then be used in conjunction with Google Maps using the supplied Image Data Lightbox software.
* Please note: these images are for illustration and do not show the exact position/angle of the mirror as it exists inside the A55/A33.