An image sensor is the device that converts light into an electronic signal. It sits behind the lens, at the focal plane. When light hits the sensor, it generates an electrical charge proportional to the intensity of the light. The camera processes that charge into a digital image. There are two main types: CCD and CMOS. CMOS is now dominant, because it is cheaper to manufacture, uses less power, and allows faster readout. CCD sensors are still used in some scientific and astronomical applications, where their sensitivity and uniformity matter.

Sensor size is one of the most important specifications in a camera. Larger sensors capture more light, which generally means better low-light performance and shallower depth of field. Smaller sensors allow smaller cameras and lenses. The number of pixels on a sensor determines resolution, but more pixels are not always better. Smaller pixels collect less light, which can increase noise. Sensor technology has improved dramatically. Back-illuminated sensors move the wiring behind the light-sensitive layer, improving sensitivity. Stacked sensors add a layer of memory, enabling faster readout and better video performance.

Sensor types

• CCD. Charge-coupled device, older technology.
• CMOS. Complementary metal-oxide-semiconductor, dominant today.
• Back-illuminated. Improved sensitivity.
• Stacked. Faster readout, better video.

The sensor is the heart of the digital camera. Its size, type, and pixel count shape what the camera can do.