The idea of the translucent mirror is that it combines the advantages of a fast SLR-type 'phase-detection' autofocus sensor with continuous live view. Previously, SLRs in live view mode had to rely on much slower 'contrast-detection' autofocus systems which used the image formed on the sensor, but in these cameras the mirror splits the image into two paths so that the phase-detection AF sensor is still active.
That's not all. The translucent mirror doesn't need to flip up to make the exposure - it's fixed in place. This allows much faster continuous shooting speeds as well as lower vibration and mechanical complexity.
In this picture of the a33, you can see the sensor 'through' the mirror (the shutter's been opened to exposure the sensor for live view). The mirror is still fixed in position, though, reflecting part of the image upwards towards the phase-detection AF sensor in the upper part of the camera.
This sounds like a terrific innovation, but Sony has taken a rather odd decision which, to some, could make the a33 and a55 start to look less like cutting-edge innovations and more like hasty kludges.
For a start, these cameras have electronic viewfinders rather than optical viewfinders. This means that the viewfinder image can display exactly what the LCD displays, including menus, icons, exposure information and so on. But this also means that the a33 and a55 have no advantage over interchangeable lens hybrids. You get the size of a DSLR (bad) but the viewing system of a hybrid (also bad).
Why didn't Sony retain an optical viewing system? This is one of the principal advantages of the DSLR design - EVFs are too grainy and low-res (even the latest high-res units like those in the Sonys) to compare with an optical viewfinder for manual focus, and they simply aren't as good at coping with extremes of lighting, either in overall intensity or brightness range. Optical systems offer the dynamic range of the human eye (huge); electronic viewfinders offer the dynamic range of a sensor/LCD display (restricted).
This is odd. There seems no technical reason why the a33 and a55 shouldn't have optical viewfinders. They might be slighty dimmer, thanks to the translucent mirror, but they would surely be better than EVFs? As it is, there seems no clear reason why anyone should buy the a33 or a55 over the NEX-3 or NEX-5.
It's true that the a33 and a55 represent a new type of camera, but the danger with trying to combine the best of both worlds is that you could end up combining the worst of them. You could say that the a33 and a55 are just hybrids with mirrors or SLRs with EVFs, both of which are backward steps.
