|Rhododendron Light, Spofford, NH|
In many of my classes and articles I have referred to the sensor on a digital camera. I have discussed its sensitivity, dynamic range and noise, all without much understanding of what goes on in that little chip of silicon behind the DSLR’s mirror. Many of my blog articles have been trigger by my own desire to learn something new, and this week it is about discovering a little of how digital sensors actually capture the images that we spray upon their surfaces.
CCDs and CMOS Sensors
At their essence, all camera sensors work the same way. The two common sensor types, CCDs (charged coupled device) and CMOS (complementary metal-oxide-semiconductor) both translate light, in the form of photons, into electrons. The electrons are amplified and create an electrical current which is constructed into an image by the camera’s software. That is an overly simplified description of a very complicated process. CCDs and CMOS sensors differ as to how the electrons are collected and amplified, and these differences affect the quality and noise of the image, as well as the cost and speed of the sensor.
Both CCD and CMOS sensors are made up of an array of silicon
|Monochrome Sensor Receive all Colors of Light|
Cambridge in Color
less power. They are also cheaper to manufacture, but because the amplifier on each pixel site takes up space on the sensor, the amount of light that can reach each photon sensing pixel is reduced. This deficiency has been largely overcome by the use of microlenses in front of each photodiode, which focus light into the photodiode that would have otherwise hit the amplifier, and not been detected.
|Microlenses Focus light around obstructions|
The new CMOS sensors are thought to perform as well our better than the conventional CCDs, but CCD sensors are still used in some video cameras and in situations where the highest quality is required such as in telescopes. Nevertheless, the improved CMOS sensors are now the used in most consumer and professional camera. They are the standard in both Canon and Nikon Cameras.
|Filters Restrict colors Received by Pixel SitesCambridge in Color|
|Bayer Array on Sensor Detail|
Most often a “Bayer” color filter array is used. This has an arrangement which includes twice as many green filters as red and blue, since the human eye is more sensitive to green light.
This is only a brief description of how digital sensors work. The complexity of the digital algorithms required to translate the data coming from the sensor into a beautifully detailed image is truly mind numbing.
Bigger is Better
|Canon 5D Mark IV Full Frame Sensor|
|Canon SX50 HS, Sensor 3% size of Full Frame|
All sensors have a certain amount of digital noise, but this is less apparent when the signal to noise ratio (s/n) is high. Larger pixels “contain” more light “signal” and therefore the images are cleaner. Finally, pixel sites that are small and jammed close together can be leak charge to neighboring sites, called “Blooming”. Larger pixel sites can contain a greater tonal range without this becoming an issue.
|5D Mark IV SX50 HS|
Small sensors have their own advantages. The cameras can be smaller and less expensive, and cropping factors expand the effective focal length of lenses. But for image quality, dynamic range and noise reduction, bigger and better pixels have clear advantages.