The newest additions to the Alpha line use Sony's Translucent Mirror Technology.
More and more interchangeable lens cameras are going mirrorless these days, but for this pair of cameras, Sony didn't nix the mirror all together. Instead, they made it fixed and translucent so that it could "split" the optical pathway, directing light to both the main image sensor and the phase detection AF sensor. As a result, shooters get full-time access to phase detection AF using the LCD for Live View or the electronic viewfinder.
