How Important is a Megapixel?

We’ve been trained to recognize megapixels as the most important thing to think of when shopping for a digital camera. It’s an easy number to keep track of, and it will better sell a camera if the "number" you're being sold is higher than the "number" you're replacing. So why is it that your new 10mp camera takes lesser-quality photos than your old 8mp camera?

Megapixel count is a way of expressing the number of tiny sensors on the chip that takes the place of film. We'll call them pixels (although that's not technically accurate -- they're actually transistors). A ‘MegaPixel’ is an expression of the number of pixels an image has. “Mega” literally means one million.

Each pixel gathers information, expressed in computer language. This information includes the relative amounts of three different colors in the light, the brightness, and the information's exact location in the picture. The more pixels, the more detail the chip can gather -- in theory. In a very general way, it is like differences in film size: the bigger the film negative, the more detail in the photograph.

Now here's the catch. The smaller the pixel, the less light it can gather. As you add pixels to a sensor of a given size, they have to be smaller. A standard 1/18, 10 MP sensor in a small camera has crowded on it twice as many pixels as there are on a 5 MP chip the same size. There is more detail available, but as it gets darker, less light falls on each individual pixel, and they begin to have trouble delivering the goods to the processor. The signals get scrambled, and we have the "noise" that we associate with digital images taken in dim light. The same thing is true of the shadow areas of a brightly-lit photo. If we try to bring out detail in the shadows, we end up with noise. If we "smooth out" the noise, we lose detail.

A sensor with fewer pixels will deliver less data overall, but all else being equal will perform better in dim light. As long as we can "get by" with 6 MP, we will get overall better results than we are likely to get with a 13 MP chip, over the widest range of conditions. Some of this is corrected by sophisticated software in the camera's processor, but the basic equation remains. Poor data, poor image.

There is so much more to camera selection than megapixels. If your choice of camera is based solely on megapixels, you probably aren’t thinking through your selection enough. Though photograph resolution is important, always look at other factors expressed in the actual image quality.