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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

image sensors
image sensors
TOP: The Olympus E-3’s 4/3rds sensor can produce outstanding images. ABOVE: The Sigma SD-14’s novel Foveon X3 image sensor captures each color on every pixel. The sensor itself has fewer overall pixels and more inter-pixel space, which helps keep noise at bay.
Various manufacturers also have taken other approaches to maximizing the amount of high-quality information gathered by the imaging sensor. Foveon developed a series of sensors utilized by Sigma D-SLRs that, unlike the imaging sensors used in all other digital cameras, records all three RGB color values for every single pixel, rather than recording a single luminance value filtered by color and interpolating the data after the capture. Fujifilm developed various imaging sensors that improve sensitivity by utilizing two photodetectors (one for normal light levels and one for bright highlights), resulting in improved dynamic range. Sony created imaging sensors that depart from the standard Bayer pattern, with red, green and blue filters over the photodetectors to instead capture red, green, blue and emerald (close to cyan). These are a handful of examples of the effort that has been put into making the most of the light striking the imaging sensor.

It’s nothing short of amazing that technology has continued to advance at such a rapid rate as to enable ever-increasing numbers of megapixels while improving overall image quality. However, as the number of megapixels goes ever higher for a given image-sensor size, overcoming the inherent limitations of smaller photodetector sizes will become increasingly difficult and, ultimately, we’ll run into barriers we can’t avoid. That will require either accepting a limitation on how many megapixels we can have ultimately or using a larger-format image sensor to enable larger photodiodes at a given megapixel count. Even before we reach whatever limits the laws of physics might impose upon camera manufacturers, we’ll likely see some reduction in the performance of new digital cameras. For example, because of smaller photodetectors, the dynamic range will be reduced, causing a loss of highlight or shadow detail in many images.

There’s no question that more megapixels lead to digital photos with improved detail and image quality. However, it has long received too much focus as a measure of the advancements in digital camera technology. What matters most, ultimately, is the quality of the final image. It’s important to appreciate that there’s much more at play here than how many megapixels are packed into the sensor. Perhaps in the future you’ll hear fewer photographers ask, “How many megapixels are in your camera?”, and more ask, “What’s the full-well limit of the photodetectors in your camera?” and “How small is the inter-pixel spacing on your sensor?” Or perhaps photographers will learn to appreciate that the technology in their digital cameras is incredibly advanced and beyond the ability of most photographers to truly understand, so they can go back to focusing on what really matters to photographers: the final image. Isn’t that what it’s all about?

image sensors
image sensors
LEFT: This illustration shows how dust and stray light are eliminated and filtered out in front of the sensor. RIGHT: You can see graphically the relative size differences between sensor types.

Tim Grey has written more than a dozen books on digital imaging for photographers. He publishes the Digital Darkroom Questions e-mail newsletter and Digital Darkroom Quarterly print newsletter. For more information, go to www.timgrey.com.


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