http://www.digitalcameraworld.com OCTOBER 2019 DIGITAL CAMERA^53
CAMERA COLLEGEConverting light into pixels
A lot happens inside your camera whenever you press the shutter release to take a picture
Colour filters
The photosites on
the sensor are colour-
blind and only detect
luminance, so in
order to create a full
colour image, each
photosite has either
a miniature red,
green or blue filter.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.)Colour filter array
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.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 green-
filtered 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.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.Photosites
Your camera’s imaging sensor
is made up of millions of light-
sensitive units called ‘photosites’.
Each of these photosites
measures the brightness of the
light that strikes it, and creates
an electrical signal in response.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.RGRGGGGBBGGGBBRRGRGRRGRGRGGGBBRGRGR