How AF WorksThe key to the SLT cameras is Sony's TMT—Translucent Mirror Technology. Traditional DSLRs present a couple of drawbacks for high-speed shooting. First, conventional phase-detection AF systems require the DSLR mirror to be in the down (viewing) position for focusing and the up (exposing) position for shooting, so the camera has to alternate between focusing and shooting. The second problem involves video. Since the camera must be in Live View (mirror-up) mode to do video, the conventional phase-detection system won't work (except to lock focus before shooting begins), so the camera must use off-the-sensor contrast-detection AF, which in DSLRs is much slower than phase-detection and not suitable for action work.
Translucent Mirror SolutionsSony's Translucent Mirror Technology provides an elegant solution to these problems: It uses a fixed semitranslucent mirror instead of the conventional DSLR moving mirror. Most of the light passes through the mirror to the image sensor, while a small portion is directed up to the phase-detection AF sensor. Thus, the camera can focus and expose simultaneously—even for video shooting.
Besides providing quick and accurate continuous phase-detection AF for still and video shooting, TMT means the SLT cameras can offer extremely quick shooting, with autofocusing for each shot, because the camera focuses and shoots at the same time, not having to alternate between shooting and focusing as conventional DSLRs do.
Conventional phase-detection AF systems require the DSLR mirror to be in the down position for focusing and the up position for shooting, so the camera has to alternate between focusingAnother TMT benefit is elimination of the conventional DSLR's bulky, complex, vibration-causing and costly viewing system (pentaprism or pentamirror viewfinder, focusing screen and moving mirror assembly). All are replaced by an eye-level electronic viewfinder and the vibration-free nonmoving mirror. While early EVFs were pretty bad, the new-tech, high-resolution, quick-refresh OLED ones used in current Sony SLTs are surprisingly good—after a brief learning curve, we had no trouble tracking flying birds with our preproduction SLT-A77 and A65 test cameras. These EVFs are certainly different than SLR optical viewfinders, but we quickly got used to them. And they make possible video shooting with the camera held at eye level, DSLR-style, rather than with the camera held at arm's length to use the LCD monitor as with conventional DSLRs in video mode (the SLR finder blacks out when a conventional DSLR is used in Live View/video mode, since the mirror is in the "up" position). The EVF can also display much more information than a conventional optical finder, and show the effects of exposure compensation, white balance and the like.
« Prev 2/2 Next