by Bob Harrington

The only question most people ask me about my digital camera is: “How many megapixels?” It is an important question, maybe the most important, but a close second is the size of the sensor, and no one has ever asked me about that.

Sensor sizes vary over a wide range but tend to fall into two general categories. DSLRs have sensors in the range from around 24x16 mm to 36x24 mm, whereas consumer or “prosumer” cameras have sensors in the vicinity of 8x6 mm. The difference is very important to the performance of the cameras because sensor size determines how much noise a sensor produces, and noise is one of the main limitations on the performance of a digital camera.

The advantages of larger sensors correspond in important respects to the advantages of medium and large format over 35 mm film. A difference is that the resolution of a digital sensor is determined by the number of megapixels, not the overall size of the sensor. Squeezing 8 megapixels onto an 8.8x6.6 mm chip, as in the Sony DSC-F828, does indeed provide 8-MP resolution, but the noise is much higher than, for example, the Canon 300D (Rebel), a 6-MP DSLR with a 22.7x15.1 mm sensor. A direct comparison of the noise of these two can be seen in the review of the F828 on the website. The F828 has about four times the amount of noise as the 300D at most ISO settings.

Noise in a digital image is manifested as speckles all over. It is worse at higher ISO settings, an understandable phenomenon when you consider that higher ISOs are achieved simply by increasing the amplification of the sensor  signals. It is analogous to turning up the volume on a radio for a distant station.

Unfortunately, you will seldom find sensor sizes in millimeters in the specifications for digital cameras. One is tempted to conclude that manufacturers intentionally obfuscate by specifying sensor size in terms such as “1/1.8,” which actually means 7.176x5.319 mm. A useful key for decoding these specs is at <>.

Prosumer cameras such as the Sony DSC-F828 or Minolta DiMAGE A2 (also with 8.8x6.6 mm sensor) are serious cameras capable of producing great images, but it would be a mistake to assume that they are comparable to, for example, the Canon 1D Mk II simply because they all have 8-megapixel sensors. There are, of course, a number of reasons why the Canon will outperform the others, but one important reason is the much larger (28.7x19.1 mm) sensor in the Canon.

This article appeared in the August 2004 Lens Paper, newsletter of the North County Photographic Society of San Diego