Converting light into pixels
A lot happens inside your camera whenever you press the shutter release to take a picture
Imaging sensor
In a digital SLR, the sensor is blocked by a set of shutter blinds and a mirror, which have to move out of the way to expose the sensor to light. In a mirrorless camera, the sensor is visible when you remove the lens.
Digital processing
The green cast is removed; a gamma curve is applied to make the image brightness seem natural to our eyes; and subtle sharpening is used to combat the softening effects of the anti-aliasing filter, if present. (The Nikon D850 shown here doesn’t have one – see the next page.)
Demosaicing
To create the full colour image, the individual blocks of primary colour have to be processed to resemble the original colours in the scene. This process is known as ‘demosaicing’. Despite a greenfiltered photosite only seeing colours that have some green light in them, it can effectively ‘see’ red and blue light by using information from neighbouring red- and blue-filtered photosites. The process uses this type of interpolation to turn the raw data into a full-colour grid of square pixels.
Photosites
Your camera’s imaging sensor is made up of millions of lightsensitive units called ‘photosites’. Each of these photosites measures the brightness of the light that strikes it, and creates an electrical signal in response.
Colour filter array Bit depth
With JPEGs, each photosite can register eight bits of data, which works out as 256 different shades of brightness. (Each bit doubles the amount of light, so 1 bit is 2 values, 2 bits is 4 values, 3 bits is 8 values, and so on.) When this is combined with the full colour information created by demosaicing, it generates almost 16.8 million possible colours.
Colour filters
The photosites on the sensor are colourblind and only detect luminance, so in order to create a full colour image, each photosite has either a miniature red, green or blue filter. In the majority of cameras, the filters are arranged in a mosaic pattern known as a Bayer filter array, named after its inventor,
Bryce Bayer of
Eastman Kodak.
The mosaic is made up of 50% green, 25% blue and 25% red because our eyes are more sensitive to green light; the end result is an image that appears sharper and less noisy than if all the colours were weighted equally. The pattern does give the image a green cast, but this is removed via white balance adjustments during digital processing.