Sony a55, a33 employ translucent fixed mirror system

The Sony alpha a55 and a33 officially debut today, bringing back the translucent, fixed mirror technology last seen in the likes of the Canon Pellix. Sony's new system boasts fast, uninterrupted auto focusing and a significant reduction in camera body size.

Sony alpha a55

For a hands-on preview complete with sample images hot off the a55's imaging sensor, check out our Sony alpha a55 First Look.

Sony calls the new system Translucent Mirror Technology. A traditional DSLR uses a mirror placed in front of the camera's sensor to bounce an image up to the optical viewfinder. When the shutter is released, the mirror is pulled out of the way, cutting off the image in the viewfinder and allowing the sensor to capture light from the lens.

In place of a traditional mirror, the a55 and a33 contain a piece of optical film. This makes it possible for light to pass through the mirror to the Exmor APS HD sensor as light is simultaneously reflected up to a 15-point phase-detect AF chip. The stream of light to the sensor and the auto focus mechanism is never interrupted.

The result is continuous auto focus, even as an image is captured. HD video recording and burst shooting mode benefit greatly from this system. As Sony claims, "Existing systems can only focus in the interval between image capture, limiting the ability to track fast moving subjects."

Without the need to accommodate a moving mirror, Sony has been able to reduce the size of the a55 and a33 significantly. The a55 is 26% lighter than the a550 DSLR, and overall size has been reduced by 23%. Both cameras will record 1080 60i AVCHD videos or 1080 30p video in MP4 format.

Sony alpha SLT-A55
The more advanced model of the two, the alpha a55 offers a maximum burst shooting rate of 10 fps. It features a 16.2 megapixel sensor and built-in GPS for easy geo-tagging.

Those three specs are the primary differentiations between the a55 and the a33. Both will feature a Sony A-mount for full compatibility with alpha lenses, a BIONZ image processor, and a 1,440,000-dot EVF with 100% coverage. Each camera's 3.0-inch 921,600-dot LCD will tilt by 180 degrees and rotate 270 degrees.

Both the EVF and LCD will display in real time the effects of whichever white balance, creative style and exposure settings are in place. The LCD and EVF will also display a digital level.

Sony alpha SLT-A33
Priced $100 lower than the a55, the Sony a33 offers a 14.2 megapixel Exmor sensor and a maximum burst rate of 7 fps. It does not have built-in GPS but retains all other features.

Like the a55, it will accept Sony's Memory Stick Pro Duo media as well as SD, SDHC and SDXC memory cards.

Pricing and availability
The Sony a55 will be introduced in October at an MSRP of $849 with 18-55mm lens or $749 body-only. The a33 will cost $749 with a kit lens or $649 without a lens and will be available in September.