From the traditional still camera side, we’re closing in on the same goal of continuous high-speed shooting. Not everyone can afford a camera like the RED Epic, but today there are a number of HDSLRs that can shoot full-resolution still images (24 megapixels, in some cases) at very high speed. For example, Canon’s EOS-1D X can shoot 18.1-megapixel images at 12 fps with AF for each shot (14 fps with focus locked and JPEG only), and Nikon’s D4 can shoot 16.2-megapixel images at 10 fps (11 fps with focus locked).
Translucent Mirror Cameras
In this article, we’re focusing on the unique technology of the Sony translucent mirror cameras, which offer a high-speed option that maintains autofocus throughout the shooting burst. Most DSLRs momentarily lose AF contact with the subject because the mirror has to flip out of the way as the exposure is made. The translucent mirror Sony models don’t have this characteristic. The AF system stays locked on because the fixed mirror provides the AF sensor with a continuous "view" of the subject.
The new full-frame Sony SLT-A99 can shoot full-res images at 6 fps (fast for a 24-megapixel full-frame camera with phase-detection AF for each shot (7 fps in cropped APS-C mode) and 10 fps in Tele-zoom Continuous Priority AE mode. The A99 also adds a new twist to the TMT story: In addition to the normal 19-point phase-detection AF system, there are 102 phase-detection sensors on the image sensor itself. This produces a much wider AF area, greatly improving tracking of subjects moving across the frame and against busy backgrounds.
The other Sony SLT cameras also give you some affordable options. The 24.3-megapixel SLT-A77 and A65 list for $1,399 and $899, respectively, and the 16.1-megapixel SLT-A57 lists for $699. Yet the A77 and A65 can shoot full-resolution, 24.3-megapixel images at 12 fps and 10 fps, respectively (at the lens’ widest aperture; 8 fps at any aperture) with quick phase-detection AF for each frame. The A57 can shoot 16.1-megapixel images at 10 fps and cropped 1.4X images at 12 fps, again with AF for each frame.
How AF Works
The 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 Solutions
Sony’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 focusing
Another 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.