While the most commonly advertised number when it comes to selling a digital camera is megapixels, image quality is not just about the number of pixels. The Nikon point and shoot Coolpix S6500, for example has 16 megapixels, but their popular entry level dSLR the D90 only has 12.3 megapixels. Which one will have better image quality? The D90 will, because resolution relies on more than just megapixels. One of the biggest factors in the quality of that final photo is the camera's sensor.
The camera's sensor is the digital equivalent of film—it collects and records all the data that makes up the image. The sensor collects light through the lens—a larger sensor will be able to collect more light, resulting in better performance in dim shooting conditions, more details in the image and a better image overall.
Making Sense of Sensors: Types
There are two main types of digital camera sensors, CCD and CMOS, and which is “better” is up for debate depending on what factors are most important to the individual consumer.
CCDs are the sensors that were first used in digital cameras. The light hitting the sensor is recorded all at once, so these sensors are less likely to produce distortion. CCD sensors are generally thought to produce images with lower noise levels then CMOS, but recent developments in CMOS sensors have changed that. CCDs are also more expensive to make and take up more battery power.
CMOS sensors are newer when it comes to the use inside digital cameras, but the technology is quickly catching up, making CMOS sensors more common. While CCD sensors are usually limited to two channels relaying the information, CMOS can have several, depending on the manufacturer and camera type, which makes them much faster, allowing for better burst capture speeds. CMOS is also catching up on low light performance. Sony, in 2009, moved the parts that transfer information from the sensor from the front to the back, creating a backside illuminated sensor that performs better in low light compared to front illuminated ones.
Making Sense of Sensors: Size
Technological advances are allowing smaller sensors to perform better, but sensor size is still one of the biggest indicators of image quality, which is why larger cameras like dSLRs perform better then small point and shoots.
Consider two cameras with the same number of pixels, but differing sensor sizes, say a small 1/3.2” and a much larger 1”. Since the number of pixels are the same, the larger sensor will have larger pixels. Larger pixels are more efficient at capturing light and detail, resulting in an image that has less noise, a better high dynamic range and a higher resolution.
The largest sensors are the full frame sensors inside professional level dSLRs, the smallest are typically inside a cell phone or tablet, something that is designed for more than just photography. Even the sensor in a high quality phone like the iPhone 5 is over five times smaller than a full frame and still smaller than most point and shoots.
In general, smaller cameras have smaller sensors—but it's not always the case. Manufacturers are continuing to find ways to pack high quality sensors in smaller cameras. Many mirrorless models have large micro four thirds sensors, but still have a more compact body because unlike dSLRs, they don't need mirrors (hence the name). Advanced compacts usually offer much larger sensors than basic point and shoots, the Sony RX100 has a one inch sensor, the Canon G1X a 1.5 inch sensor, both a substantial change from the size of a basic compact.
Comparing two different cameras based on sensor size is not so easy, however. Manufacturers don't list the sizes the same way. Some may be listed in millimeters, others in ratios. To compare the two, make sure you are comparing apples to apples and that both are in the same format. If the two are not in the same measurement, Sensor-Size.com is a good resource for quickly translating the numbers for a better comparison.
Making Sense of Sensors: Conclusion
Sensor size and type is only one aspect to consider when choosing a new camera, but it's a large predictor of image quality. Sensor size is particularly important for low light photography and for enlarging images. Technological advances continue to improve on sensors, making smaller sizes more efficient and the different types more equal in terms of performance.