Sony's latest interchangeable lens cameras, the SLT Alpha A33 and A55 represent a significant technological milestone - not just for Sony but for the enthusiast camera market as a whole.
The company has rejected the traditional DSLR design and instead created a hybrid that, like a compact camera, is from the ground up built around live view, but one that is also capable of offering full-time DSLR-style phase-detection autofocus. The combination means they can offer features such as phase-detection AF during movie recording and extremely fast continuous shooting rates (10 frames per second on the A55), previously unthinkable at this price.
This is made possible by adopting an approach that has more in common with a mirrorless camera (like the Panasonic G2, for example) than an SLR by removing the bits that pretty much define such cameras: the optical viewfinder and moving mirror.
The designation 'SLT' stands for single lens translucent and it's the 'translucent' bit that's the key to what differentiates these new models both from conventional DSLRs and mirrorless interchangeable lens cameras. The SLTs do have mirrors, but they're mirrors that let the majority of the light pass straight through to the sensor, rather than having to swing out of the way to allow exposure. As a result they are fixed in position, always reflecting a portion of the light emerging from the back of the lens onto a phase-detection AF array housed in the top of the camera. (A newly-developed 15-point array in the case of these two cameras).