Sony’s latest APS-C cameras, the SLT-A55 and SLT-A33, look like conventional DSLRs, but feature a fixed pellicle mirror and an eye-level electronic viewfinder in place of the DSLR’s eye-level optical viewfinder. The system allows for quick, continuous phase-detection autofocusing in Live View mode, even for video recording, in a way not possible with traditional mirror design.
In a conventional SLR, the mirror sits in the “down” position for viewing, directing the image up to the focusing screen and on to the pentaprism (or pentamirror) eye-level viewfinder. When you press the shutter button to take a shot, the mirror flips up and out of the light path, allowing light to reach the image sensor (or film, with film SLRs). The mirror returns to the viewing position immediately after the exposure has been made.
There are some drawbacks with the traditional approach. First, the viewfinder image blacks out during exposure, since the mirror is no longer in the viewing position. Second, the mirror’s movement causes vibrations, which can adversely affect sharpness with high-magnification and long-exposure images. Third, it creates problems with DSLRs that offer live-view operation. The quick, accurate phase-detection AF systems used by DSLRs require the mirror to be in the down position for light to reach the AF sensor. But the mirror must be in the up position for light to reach the image sensor and produce the live-view image. When phase-detection AF is used in Live View mode with conventional DSLRs, the live view is disrupted while the camera focuses.
To eliminate this problem, DSLR manufacturers also include contrast-based AF right off the image sensor during live view. The problem here is that contrast-based AF historically has been much slower than phase-detection AF, making it unusable for action subjects and decisive moments.