Photons, Pixels, Photoelectrons, and Digital Photographs

Digital cameras have revolutionized photography in a major way!  Instead of taking a few rolls of film with 24 slides per roll and having to wait until the film was developed to see our photographs, we can now take thousands of photos in a day and see the results instantly.

Most photographers treat their digital camera sensor like a “black box”, not really wanting to learn how it works.  Sometimes a little bit of knowledge goes a long way.  This is the case here.  Learning some of the basics of camera sensors will help improve your photography.

Your camera sensor is an array containing millions of photosensitive sites, called pixels.

When light, photons, land on a pixel, photoelectrons accumulate in the pixel.  Photoelectrons are converted to a voltage for each pixel, amplified, and transmitted as an analog electrical signal.

The electrical signals are digitized by an analog-to-digital converter (ADC), so they can be recorded on your memory card — as an integer number for each pixel.  This makes your camera a “digital” camera — the image information is stored as a digital (binary) number.

A staggering amount of data is measured and recorded at high frame rates.  A 20-megapixel camera that takes ten photographs per second reads and stores at least 200 million pixel values per second.


Sony a7R III digital camera sensor is shown here.  This is a 42.4 Megapixel full-frame (36 mm x 24 mm) sensor that uses a 14 bit ADC to digitize the electrical signal from each pixel.  The shutter can fire at 10 frames per second.

(Photo courtesy of Tony Webster from Minneapolis, Minnesota, United States, Creative Commons 2.0 license)

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