The Camera Sensor

Cutaway view of camera sensor

Amazingly, the digital sensor in your camera works very similar to the retina in the human eye.  Light enters the lens of the camera and is focused on the image sensor.  A small voltage is applied to the sensor when the camera is triggered and when a photon of light strikes the sensor plate it is registered in the camera.  The more times the sensor is struck, the brighter the image is registered in the memory.  The sensor is actually made up of individual microscopic sensors arranged in a grid.  Each sensor is given a filter of either red, green or blue, so if you took a photo of a red apple, the sensors with the red filter would register the light and create an image of a red apple.

Sensor filter grid pattern (greatly magnified)

The sensor can me made more or less sensitive to light by changing the ISO setting of the camera, which in turn varies the voltage that is input to the sensor plate.  One reason for increasing the sensitivity might be to compensate for low light levels, however if the sensitivity is increased too much, the photo develops "noise". The science behind the actual cause of digital noise is complex and varies from sensor to sensor, but it is usually best to avoid noise if possible as it is difficult to remove and degrades the image.  The threshold for noise in you camera depends on the brand and age.  Ten years ago any ISO over 200 had an un-acceptable level of noise, where newer cameras have acceptable noise levels up to 6400, and in time that will change for the better.

Image with digital noise