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Sizing a Sensor: No Easy Way

This week, you get two columns in one.

One is about new pocket cameras that take sharp pictures in low light without the flash — a magnificent moment in the evolution of cameras, thanks to an unusually large light-sensing chip.

The other column is about a shady scheme that’s being perpetuated by the world’s camera companies.

If there’s one single statistic that you can use to compare cameras, it’s sensor size. A bigger sensor soaks up more light. You get better color and sharper images, especially in low light. A big sensor generally means better color and clarity, and less grain and blur in low light. Digital S.L.R. cameras have enormous sensors, which is why professionals use them. (Of course, S.L.R.’s are also enormous and heavy.)

But when you’re shopping, how do you find out the sensor size? It’s not on the box. It’s not in the ad. You can Google it (“Rebel XT sensor size,” for example). But the information you will find is mostly worthless.

First, sensor sizes for S.L.R.’s are expressed in millimeters, width by height (23.7 x 15.6 millimeters), not inches diagonal. Yet sensor sizes for pocket cameras are expressed differently, believe it or not, as ratios, like 1/2.3 inches. You can’t easily compare S.L.R. sensors with pocket camera sensors and you can’t compare pocket cameras without a calculator.

At sensor-size.com, you can convert every sensor format into inches diagonal. But recently, I learned something scary: even that isn’t the nastiest part of the sensor-size shenanigans.

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It turns out that even if you divide out the 1/2.3 thing, the result — 0.435 inches, for example — does not represent the sensor’s real size. Those decimal fractions don’t measure the sensor. Instead, because of a bizarre 50-year-old convention, they measure the 1950s television tube that those rectangular sensor chips could fit inside of. And that’s the outside diameter.

In other words, the actual sensor size is much smaller than what the camera companies publish — about one-third smaller. A camera with a 1/2.7-inch chip does not measure 0.37 inches diagonal — 0.38 is the size of the tube it would fill. The actual sensor is much smaller: 0.26 inches. (The full explanation is here —www.bit.ly/fQw37l. I confirmed the information with Canon.)

This might sound awfully picky, but it’s not. It’s like finding out that the TV industry has been exaggerating its screen size for 50 years.

The camera companies say that they use this absurd measurement not to fool you, but out of many decades of habit. Well, it’s time to change.

In the early digital camera days, every company measured battery life differently. Finally, the Camera and Imaging Products Association (the Japanese camera trade group) proposed a standard, realistic testing protocol. Today, shots-per-charge is measured identically by every brand, and you can rely on that figure. Well, guess what? For its next trick, CIPA should clean up this sensor-size mess — soon.

For now, you can use those decimal fractions only for comparison purposes. A 1/1.7-inch sensor is definitely bigger than a 1/2.8-inch sensor, even though those aren’t the real measurements.

What the world has always wanted is a big sensor in a small camera, so you can get sharp photos in low light without hauling around an S.L.R. This year, the camera industry took a big step toward that glorious future. Canon’s PowerShot S95 ($370), Panasonic’s Lumix DMC-LX5 ($400) and Samsung’s TL500 ($370) are all pocket-size cameras — with sensors at least 50 percent larger than other pocket cameras.

(The Canon and the Samsung have 0.59-inch sensors, while the Panasonic is calculated to be 0.61 inch. Most pocket cameras’ sensors are about 0.37 inch. All of these, of course, are the misrepresented “tube” measurements; the real diagonals are about a third smaller.)

The result is one reliably spectacular photo after another — especially in low light without the flash. The sensors aren’t as big and the photos aren’t as good as what you get in an S.L.R. But they’re halfway between a pocket camera and an S.L.R.

Professional photographers are snapping up these pocket-size wonders as secondary, always-available cameras. But if you can afford it, you too should opt for these cameras instead of the usual mass-market cheapies.

All three of these cameras also have amazingly “fast” lenses. That’s camera-speak for “they let in a lot of light,” which also helps in low light. The Canon and Panasonic have f/2.0 lenses, which let in twice as much light as the typical f/2.8 lens. The Samsung is even better, with an f/1.8 lens — an incredibly rare spec. Those wide apertures also mean that these cameras can sometimes deliver the blurred-background effect that an S.L.R. produces so easily.

As you can see from the slide show that accompanies this column online, the results look darned near professional.

The Canon S95 is the smallest of the three. If you know what you’re doing photographically, you’ll love the big, clicky ring around its lens barrel. It lets you make quick adjustments without burrowing into menus. Which adjustment? That’s up to you. With one button press, you can redefine the ring’s function: zoom, manual focus, exposure adjustment, white balance, ISO (light sensitivity), shutter speed or aperture.

The S95 also has a few ingenious improvements on the standard self-timer. There’s Smile Shutter (waits to snap until you smile), Wink Shot (waits two seconds after you wink) and Face Detect (waits two seconds after a new face enters the frame — yours, that is, after you’ve set up the shot and rejoined the group).

The photos are almost universally fantastic, indoors and out, and so is the hi-def video. A shame about the feeble battery life: only 200 shots a charge.

That’s half what you get from the Panasonic LX5. The LX5 is a bigger, heavier camera, too. And it has a detachable lens cap, which you’ll lose.

But this camera, too, has some fine features. For example, it has an especially wide lens, capable of capturing much wider vistas than its rivals. The dedicated video button lets you record hi-def movies without wasting time changing modes. It has a satisfying physical on-off switch, and the built-in flash pops up only when you push another switch. The hot shoe on top accommodates a more powerful flash or an eyepiece viewfinder.

The Samsung TL500 is bigger yet. At 4.5 by 2.5 by 1.1 inches, it’s halfway into the size/weight territory of the Canon G12 and Nikon P7000 — big black things loaded with controls but heavy as bricks in your pocket.

This camera has a lot in common with the Panasonic: hot shoe, detached lens cap, superwide-angle lens. The best part, though, is the huge, bright and clear OLED screen. It flips and pivots away from the back so you can shoot over your head, down at knee level or even forward, so you can frame a self-portrait without guessing.

Unfortunately, the Samsung records only standard-definition video. The zoom is 3X (the Canon and Panasonic manage 3.8X). And in too many shots, the Samsung’s white-balance setting is off, giving photos a weird color tinge. Note, too, that you must charge the battery inside the camera, so you can’t charge a spare while shooting.

Now, a real S.L.R. is still superior in many ways — no shutter lag, interchangeable lenses and easier background-blurring. But if you want pocket-size, these three remarkable models stand head, shoulders and torso above the typical $200-ish compacts.

Here, then, are two New Year’s resolutions. For the camera companies, clean up your deceptive, lazy sensor-measuring act. There’s no measurement as important as the sensor dimensions, and the public deserves to know what it’s buying.

And for you, the consumer, consider what you really want. If the answer is better pictures, start saving up. There’s still no such thing as an S.L.R. in your coat pocket, but there’s now something that comes breathtakingly close.

David Pogue is a columnist for the New York Times and contributor to CNBC. He can be emailed at: pogue@nytimes.com.