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.
Sony’s Translucent Mirror Technology addresses these limitations with a fixed pellicle mirror that transmits most of the light—but also reflects part of it. The transmitted portion goes through to the image sensor, providing the live-view image and making the exposure. The reflected portion goes up to the phase-detection AF sensor, allowing full-time phase-detection AF during live view with no disruption of the live image. The SLT cameras can focus continuously and shoot at the same time, even in video mode.
The SLT system offers a number of other benefits, as well. It makes for quicker shooting, eliminates vibration caused by mirror movement and does away with the brief viewfinder blackout during exposure that occurs with conventional SLR cameras while the mirror flips up and out of the light path so light can reach the image sensor. (Note: While the SLT mirror doesn’t move during shooting, you can manually move it to the up position when desired to clean the image sensor.) The SLT cameras can use all Sony and legacy Konica Minolta Maxxum lenses, no adapter required.
Putting a pellicle mirror in an SLR has been done before. Canon offered the 35mm Pellix back in 1965, with a fixed pellicle mirror and the selling point of no viewfinder blackout while shooting (as well as no mirror vibration, less weight and minus the cost of a moving mirror mechanism). In 1972, Canon introduced the pro F-1 High Speed Motor Drive Camera, with a fixed pellicle mirror that enabled a top shooting rate of 9 fps—you could blow through a 36-exposure roll of film in four seconds! That was followed by the new F-1 High Speed Motor Drive Camera in 1984, which could do 14 fps. In 1989, Canon introduced the EOS RT, with a fixed pellicle mirror that reduced shutter-release lag time to just 0.008 seconds—the first auto focus pellicle-mirror SLR.
Sony’s new SLT-A55 and SLT-A33 are the first digital SLRs to employ pellicle mirrors. Well, they look like DSLRs, but do away with the pellicle-mirror dim-viewfinder problem (and pentaprism/pentamirror finder bulk) by employing an eye-level electronic viewfinder instead of an optical SLR finder. Because the mirror doesn’t have to move, the camera can focus and shoot simultaneously for super-quick operation. In Continuous Priority AE mode, the A55 can shoot 16-megapixel images at 10 fps, while the A33 can shoot its 14-megapixel images at 7 fps, with continuous autofocusing and metering. This translates to pro DSLR speeds in sub-$1,000 cameras.
Another advantage of the SLT cameras is that you have convenient eye-level viewing when shooting videos—something the moving-mirror DSLRs don’t provide (their eye-level optical finders black out during live-view operation).