Megapixels vs. sensor: does size matter?

With camera companies rolling out cameras with higher and higher resolution, and only subtle upgrades to zoom and focal length, we have to ask -- how important are megapixels?

Consumers seem to find them compelling, with 70 percent of the nearly 23 million households that upgraded cameras in 2008 seeking "more megapixels and new features," according to the Consumer Electronics Association (CEA). But does a 12-megapixel camera really offer better picture quality than a 10-megapixel camera? Not necessarily. Megapixels alone will not help you take a better picture, though higher resolution enables you to make larger prints. Better image quality really comes down to sensor size, a spec that is rarely mentioned at flashy press conferences.

In his article "Pixels Are Like Cupcakes, Let Me Explain," for The New York Times, Russ Juskalian describes the sensor mechanics as a "flat sheet of material pocked with millions (hence "mega") of cylindrical, cuplike pixels. In other words, picture the digital sensor as a tiny cupcake tin." Juskalian notes that while megapixels have increased exponentially in the past few years, sensors have not. He goes on to explain that cramming more pixels into the same-sized sensor means that pixels are actually getting smaller and thus capturing less light, resulting in noisy images.

So if you're comparing an 8-megapixel camera with a 10-megapixel camera, see what size each sensor is. If they're the same size, you're better off with the 8-megapixel camera, which will have better picture quality, with less image noise, and most likely a lower pricetag. According to a comparison chart at sensor-size.com, the average 10-megapixel camera has a 0.43 inch (1/2.3 inches) sensor, while an 8-megapixel camera has an average sensor size of 0.40 inches (1/2.5 inches).

Digital camera specs, including sensor size, frame and focal length, are often compared to 35 mm film cameras. You may have heard the term "full frame sensor" in relation to high end digital SLR cameras. A full frame sensor has no crop factor compared to a 35-mm lens. DSLRs with full frame sensors cost $2,500 and up and are rather large in size. Your typical compact camera ($200-$300), no matter how high the resolution, will not have a full frame sensor. Bob Atkins at Photo.net offers a great visual interpretation of sensor size that shows the vast difference between three cameras that all share a 3-megapixel resolution.

Editors at CambridgeInColour.com, a photography enthusiast site, describe crop factor as "the sensor's diagonal size compared to a full-frame 35 mm sensor." They go on to explain that crop factor is not necessarily a bad thing when it comes to your standard point-and-shoot because "nearly all lenses are sharpest at their centers, while quality degrades progressively toward to the edges. This means that a cropped sensor effectively discards the lowest quality portions of the image." Many experts note this issue when reviewing point and shoot cameras, describing sharp centers and soft edges in shots.