Sony's latest digital SLR models, the and the Alpha DSLR-A350, introduce an innovative, dual-sensor approach to previewing your image in real-time on the LCD screen, just as you can do today with digital point-and-shoot cameras.

Although Sony is not the first company to use two sensors to generate a live preview, the company is the first to offer both live preview and full control of the auto-focus system, including during rapid-fire burst-mode shooting.

State of Live View

Live view remains a relatively new feature on digital SLR models. By definition, with a digital Single Lens Reflex (SLR) camera, you see what the lens sees; but, the actual image is captured when you flip up the mirror and open the shutter. In order to offer a live preview of an SLR model, manufacturers have typically had to preview the image off of the same sensor that handles capturing the image.

The first digital SLR with a live LCD preview -- Olympus E-330 -- came out about a year ago. Now, several other manufacturers -- including the Canon EOS 40D and EOS 1D Mark III, and the Nikon D300 and D3 -- are offering this feature; and the recently introduced Pentax K20D that does so as well.

The challenge, says Mark Weir, Sony's senior technical and marketing manager for cameras, was to come up with an approach to live view that was affordable to a price-sensitive consumer, but also dealt with the sluggish speed issue.

Most previous attempts at live preview have used the camera's imaging sensor to also create the live preview. Sony set out to create a sensor design in which a live preview would not interrupt the fundamental operation of the camera--and therefore wouldn't impact the auto-focus system or the capture speed. Sony's solution: introduce a second sensor dedicated to creating the live preview. Sony isn't the first to use a dual-sensor approach, but it is the first to introduce support for burst-mode live preview. Olympus' E-330 also used two sensors.

Image Capture: How it Works

Because of the way the Sony cameras are designed, says Weir, the shutter, the mirror, and the focusing system are unaffected by whether you are in the live view mode or in the optical view finder mode. That the auto-focus system is unaffected is critical: Some competing models are unable to effectively auto-focus when in live view mode. As explained by Weir, competing cameras must interrupt the live view in order for the focusing system to work. While the mirror is up, the camera's phase detection focusing system (which uses a passive autofocusing technique) cannot work.

With Sony's implementation in its new cameras, you simply half-press the shutter to focus, and then fully depress the shutter take the picture.

The secondary sensor creates the live view, so the mirror on the A-300 and A350 doesn't lift until the moment of exposure, to capture that image. As such, the focusing system is not interrupted.

Burst Mode Shooting

This approach means that you're not losing speed, either. Another unique feature, according to Sony: The ability to have a live preview in between the shots of continuous shooting, such as when you're trying to capture a baby crawling or a baseball player sliding home.

"It's almost impossible to have live view in burst shooting today; to change the state of the imager from one mode to the other takes too long to practically create a preview image in between the images of a burst," says Weir. "But with a two-sensor system, we have restored the ability to build a live preview in between shots in a burst."

That said, don't expect to fire off a burst of images in live view mode. "In a live preview, it's not practical to maintain its full 3 frames per second burst speed; the image would need to be under 1/3 of a second in time," says Weir. "Practically, we felt that didn't make sense. In live view mode, the camera's frame rate is reduced to 2 frames per second. Otherwise, the preview wouldn't be particularly useful [since the images will go by so quickly]."