Instead of Single Lens Reflex (SLR), the new Sony α55 and α33 are SLT (Single Lens Translucent mirror) cameras. That’s not just a marketing phase. It is actually quite interesting feature considering it offers 10 frames per second shooting (α55 only. 7fps in α33) that is typically only found in professional grade Nikon and Canon SLRs.
According to Sony, Translucent Mirror Technology uses a fixed, translucent mirror that splits the lights coming into the lens between the main image sensor and a separate phase-detection auto-focus sensor. What it means is that you get the best of the both: high-resolution live image preview, and fast accurate phase detection auto-focus that are available at all times either via the swirl and tilt 3″ LCD or an electronic viewfinder. With a fixed mirror, the mechanical design is simplified to allow smaller body and less complex system that cost less and are more durable.
In a traditional SLR camera, there is also a translucent (semi-transparent) mirror. The light is also split between the AF system and the viewfinder. But the similarity ends there. When the shutter release button is pressed, the mirror moves out of the way to allow exposure on the film or image sensor. When the mirror is up, the phase detection AF is disabled and the viewfinder is blacked out. In the Sony α55 and α33 the mirror is fixed so both the (electronic) viewfinder and the contrast detection AF are available at all times.
This is actually not the first time a fixed semi-transparent mirror (called Pellicle mirror) is used in a camera. Canon used it in the 1960s. If you are interested in knowing more, this CNET Asia article has a good summary of the technology. The article made a mistake by saying DSLRs do not use Pellicle Mirror. They do, just they have the conventional up-and-down type.
The technology isn’t without drawbacks. Some of the light is always diverted away from the image sensor so there is reduced overall low light sensitivity. It is interesting to see how market will react to Sony’s α55 and α33. I would not expect these new products will enable Sony to get close to Canon and Nikons number 1 & 2 position in DSLR market any time soon.
Keywords: SLT, Sony, Translucent Mirror Technology