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Monday, September 1, 2008

Megapixels: How Much Is Enough?

Pixel count certainly plays a key role in photography, but we’re approaching a point of diminishing returns and potentially reduced image quality as more pixels are packed onto a sensor

This Article Features Photo Zoom

This cutaway drawing gives you a sense of how much technology there is to making the photograph besides just the sensor.

It’s time for the “megapixel wars” to end once and for all. You can bet the march of technology will continue to give us imaging sensors with ever-greater numbers of megapixels, but for most photographers, more pixels won’t give us better images. Sure, more megapixels generally are welcome. But the reality is that we’ve reached the point where most photographers won’t benefit from more megapixels.

It’s understandable that the issue of image-sensor quality was largely distilled down to how many pixels were on the sensor, considering that these technological gems are incredibly complicated. But when you hear it stated that way, it reinforces how much we’ve been collectively ignoring other aspects of image-making technology by focusing, in large part, on a single specification. Now that photographers as a whole seem to realize that megapixels aren’t the key feature of their imaging sensors, it makes sense to examine what else is going on behind the pixels.

I’m not suggesting that photographers have been ignorant of their imaging sensors. To be sure, photographers have grown increasingly savvy about the technology inside their cameras. Many photographers, for example, have debated the merits of CMOS- versus CCD-based imaging sensors.

At the core, an imaging sensor exists to capture light. Because light is energy, an imaging sensor uses an electrical charge as the measure of the intensity of light. That measurement is the job of the individual photodetectors arrayed on the imaging sensor, which together combine to create the photographic image that results from considerable processing by the camera (and potentially by a computer after the capture). Understanding this provides some perspective on how a digital camera produces a photographic image, but it glosses over the considerable amount of technology at play.

ABOVE: Image sensors from a variety of manufacturers.

The imaging sensor is the collection of all the photodetectors that record the light coming through your lens, among many other electronic components contributing to the overall process. The sensor has full responsibility for the amount of detail rendered in a scene. The structure and arrangement of photodetectors and supporting electronics and components determine how much information is captured when you press the shutter release. You can perform all sorts of tricks in the conversion from a RAW capture and with optimization techniques in Photoshop, but the result never will be more information than was captured by the imaging sensor. That makes the sensor a pivotal player in producing a great photograph, second only to the photographer taking the picture.

For the most part, digital cameras have taken the path of evolving from film cameras, with some obvious, and significant, differences. As part of that heritage, most digital SLRs have imaging sensors that match the aspect ratio of a frame of 35mm film, and in many cases, the sensor is the same size as that frame of film. Regardless, the sensors tend to be one of a handful of fixed sizes, which is important from the standpoint of achieving consistent behavior for a given lens. If the size of the imaging sensor was changing with each new camera model, you’d also need a new set of lenses with each camera to achieve the expected results.

Because every iteration of an imaging sensor for a given line of cameras tends to have the same overall dimensions, increasing the number of megapixels involves an important requirement—as you increase the number of megapixels, the individual photodetectors must get smaller. That’s not exactly a revelation, but it isn’t a trivial matter either. The size of the individual photodetectors plays a huge role in the final image quality. It determines the maximum electrical charge they’re able to measure. Keep in mind that electrical charges consist of electrons, and while they’re certainly tiny (physicists haven’t yet determined the actual size of an electron, so suffice it to say it’s very small), they consume space.