Chip size vs noise

In general, imaging chips come in three sizes: Full-frame chips are those that closely, even exactly, match the 36 x 24 mm frame size of 35 mm film. Some digital SLRs use the APS-H sized sensor, which measures 29 x 19 mm; others more commonly use the APS-C sized sensor, which measures 22 x 15 mm. Other cameras use the Four Thirds system developed by Olympus that is now being used by other manufacturers, including Panasonic and Leica, and is smaller than most other digital SLR sensors. The size of this sensor is 18 x 13.5 mm, and it has an imaging area of 17.3 x 13 mm.

When it comes to image quality and reduced noise, size does matter. On the left is the full-frame chip from the Canon EOS-1Ds Mark II. Next is the imager from the Canon EOS-1D Mark III, and finally there's the chip from the EOS Rebel Xti. Images courtesy of Canon USA.

When it comes to image quality and reduced noise, size does matter. On the left is the full-frame chip from the Canon EOS-1Ds Mark II. Next is the imager from the Canon EOS-1D Mark III, and finally there's the chip from the EOS Rebel Xti. Images courtesy of Canon USA.

This is the original Kodak Four Thirds CCD sensor used in the initial camera offerings from Olympus. More-recent cameras introduced by Olympus have used a Panasonic NMOS sensor mounted to a flexible circuit board. It looks a bit different, but is the same four-thirds size. Photo courtesy of Olympus America.

There are lots of debates about the advantages and disadvantages of using large, full-size (24 x 36 mm) imaging chips in digital SLRs, but the one obvious advantage of full-frame sensors is the ability to combine high resolution with large pixel size. Remember that each pixel on an imaging sensor transmits an electronic signal only in proportion to the brightness it receives. In effect, a pixel is acting as a tiny light meter. A larger pixel has a greater surface area available for gathering light. More light collected means that less amplification is needed for the output signal of each pixel, and less is better because magnifying low-level signals increases noise. For example, if the chip's base area is five times the average, then light-gathering and light-storage capacities also increase fivefold. In the extreme case of low-light photography and ISO ratings of 800 or faster, high signal-to-noise ratios give full-frame sensors a greater advantage.

After 40 years of using another brand (read about it in the first edition of this book), Barry recently switched camera systems to Canon. One of the main reasons for the switch was Canon's ability to produce better-quality image files at higher ISOs with less noise. Nowadays, he confidently sets the camera ISO to 800 and higher without worrying about image quality. © 2007 Barry Staver.

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