Revolution in Single Lens Reflex Camera

Just by looking at it, you'd never guess that Sony's new Alpha A300 digital camera represents a huge technical breakthrough. To discover what it is, you need a tour of its innards. Keep hands and feet inside the tram at all times.
On an ordinary single-lens reflex camera (those big black pro models), light enters from the lens and is split by a semi-transparent mirror. Part of the light goes to the eyepiece viewfinder, and the other part goes downward to the autofocus sensor.
When you press the shutter button, that mirror flips up out of the light's path, revealing — aha! — a small rectangular image sensor, the computer chip that records the photo.
Already, you've learned enough to answer one of the great digital camera mysteries: Why must you hold these cameras up to your eye? Why can't you frame a photo using an S.L.R.'s back-panel screen, as you can on a little pocket camera?
Actually, a few recent S.L.R. models do, in fact, have this Live View feature, but it's mostly a disaster. It works by flipping that mirror up out of the way, so that light from the lens hits the image sensor, which feeds the image to the screen. Trouble is, once the mirror goes up, no light hits the autofocus sensor, so the camera can't focus.
So here's what happens when you press the shutter button. There's a noisy clank as the mirror drops down again; the screen goes black; the camera computes focus and exposure; the mirror lifts again; the screen comes back to life; and finally — a second or so later — the shot is recorded.
In other words, Live View on existing cameras is slow, noisy and deeply confusing. All of this silliness arises because the camera's image sensor must do double duty: it's responsible for supplying the screen with a live preview and for recording the shot.
Sony's technical breakthrough on the A300, therefore, was this: "Duh! Put in another sensor!"
On this camera, turning on Live View sends light from that main mirror onto a second sensor, one that's devoted solely to feeding the preview screen. The autofocus sensor works normally as you compose a shot, since the mirror never has to flip up.
As a result, Live View is a completely different experience. The camera focuses quickly as you aim the lens, without ever blacking out the screen. When you press the shutter, the screen doesn't go on-off-on, there's no loud clacking, and there's no baffling exhibition of mirror calisthenics inside the camera.
In this regard, using the A300 can feel a lot like using a compact camera. That's precisely what Sony was hoping; it's aiming this 10-megapixel model ($763 with starter 4X zoom lens) at people who are graduating from pocket cameras to something more serious.
(For about $140 more, you can buy this camera's 14-megapixel sibling, the A350. Don't fall for it. Ten megapixels is already enough resolution for prints the size of a minivan; 14-megapixel photos just eat up your hard drive faster and slow the camera. Furthermore, if you do the math, you'll discover that a 14-megapixel photo contains only 18 percent more pixels in each dimension than a 10-megapixel shot — not 40 percent, as instinct might suggest.)
Now, serious photographers traditionally snort at the whole idea of composing shots on the screen. Only an eyepiece viewfinder shows the true scene as the camera sees it.