Sensors Exposed
More than just megapixels—what you need to know about your digital camera’s core component
| Text and Photography By Budd Riker |
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At the heart of every digital camera is an image sensor, a silicon chip that contains millions of tiny light-sensitive photodiodes. Each photodiode produces a pixel of the captured image, and the number of pixels (resolution) is the horsepower spec that gets the most attention. However, the quality of the final image isn't determined by the number of pixels alone. When comparing cameras and their sensor specs, you need to do more than merely count megapixels-there's a lot more about sensors that you'll want to consider.
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One way manufacturers make little pixels collect lots of light is by placing microlenses over them. The microlenses collect light and focus it on the photodiode. Making the microlenses larger and decreasing the gaps between them increase the light-gathering area of each pixel. |
When you trip the shutter in a D-SLR, the image from the lens is projected onto the image sensor, just as it's projected on film in a 35mm SLR. Each pixel is like a little bucket that collects photons (light particles) in proportion to the brightness of the point in the image it records: pixels recording bright areas collect more photons; pixels of dark areas receive fewer. The sensor then transforms the collected photons into electrical charges, which are converted into voltages that can be read by the A/D converter. The A/D converter translates the analog data (yes, image sensors are analog, not digital devices) into digital data that can be used by the image processor.
Pixels
Digital images are made up of tiny squares called pixels (short for "picture elements"). A megapixel is one million pixels. A 10-megapixel camera produces images made up of approximately 10 million tiny squares. For a sensor with a 3:2 aspect ratio, that's typically 3,872 pixels by 2,592; for a Four Thirds sensor, that's 3,648 by 2,736. The 10-megapixel digital camera typically will have an image sensor containing the same number of light-sensitive photosites: 3,872 across by 2,592 down (or 3648 x 2736). In actuality, image sensors contain slightly more photosites than are used for the image, so both gross (total) and effective (used for the image) pixel counts are often given in camera specs. Note that this 1:1 photosite-to-image-pixel relationship doesn't apply to the unique Foveon sensor.
Keep in mind that there's a lot more to image quality than just the sensor's pixel count, however.
The sensor, image-processing engine and A/D converter work together to determine the dynamic range, color reproduction, noise level, sharpness and more.
Pixel Size
All other things being equal, more pixels mean higher resolution, and bigger pixels mean better
image quality-the bigger the light-capturing area, the wider the dynamic range, the greater the signal-to-noise ratio, the higher the top ISO speed and the less the blowout due to oversaturated pixels. For example, the full-frame (36x23.9mm) 12.1-megapixel CMOS sensor in Nikon's D3 produces better image quality and higher usable ISOs than the APS-C (23.6x15.8mm) 12.3-megapixel CMOS sensor in the Nikon D300, mostly because the pixels are 2.4x the size. (For the record, the D300 delivers terrific image quality, just not as good as that of the D3.)
When comparing pixel size, be aware that two sensors using same-sized pixels might have different fill factors: one might devote a greater portion of each pixel to light-gathering and thus produce better results than a sensor with the same-sized pixels, but with a smaller portion of each pixel devoted to light-gathering. And image quality is a product of not just the pixels on the image sensor, but also the technology in those pixels, the A/D converter, the image-processing engine and the lens-and the way they all interact.
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