In what is--seriously--the worst-kept secret of the summer, Sony officially announced the first members of its innovative new series of cameras, the Alpha SLT-A33 and SLT-A55V, which are the first cameras to incorporate translucent mirrors. I, along with about 15 other reviewers, got a chance to shoot with the cameras--as per our policy CNET footed the bill for my trip rather than Sony--and have sample photos and some preliminary analysis of the photo and video quality and ergonomics of the cameras.
Also known as a pellicle mirror, a TM passes most of the light from the lens through a fixed semitransparent mirror, reflecting a small bit of the light upward to a separate phase-detection autofocus sensor. This is how Sony achieves the faster phase-detect continuous AF for movie capture, while most current interchangeable-lens cameras (ILCs) and dSLRs use the slower contrast AF, which is based off the imaging sensor.
One doesn't necessarily need to use a mirror to incorporate phase detection, though; for example, Fujifilm's recent F300 EXR and F800 EXR point-and-shoots use a phase-detection array layered over the image sensor. Because many older dSLR-mount lenses can only work with phase-detection AF--that's why ILC adapters for older lenses generally don't support AF--Sony's system enables autofocus when using those lenses for shooting video.
Most traditional lenses have noisy focusing and aperture activation mechanisms, however, which makes them unsuited for video AF. Sony says it focuses so quickly that the noise should be barely perceptible, and that anyway you should use one of the new dedicated microphones (models ECM-ALST1 and ECM-CG50) for better separation from the lens. I didn't get a chance to test Sony's audio claims. The quieter lens is one of the rationales behind the electromagnetic activation in the NEX series' E-mount lenses, and it's hard not to feel like Sony's entering some confusing territory by pairing its most video-oriented still camera with its old-fashioned lens system. The cameras also update to a 1200-zone metering system.