Here is the quote from the most famous dpreveiw for a55:
"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 1/3 EV reduction in light transmission for live view/image capture but means the AF sensor is always receiving light."