How do digital cameras work? The camera is
one of those wondrous inventions that changed the world. Before it arrived the only way
to make a picture was to draw it or paint it. Which obviously removed a fair amount
of the spontaneity when you were trying to knock out a quick selfie of yourself giving
epic duckface while eating a Maccy-Ds with your bezzie mates. Which is probably why Rembrandt always looked
so depressed in his self-portraits.
Anyway, the camera made it possible to make an instant
picture in a fraction of a second. But the process involved in getting the image out
of the camera and printed was a complicated one. The light-sensitive film had to be carefully
removed, sent to a processor, get developed, turned into a negative and then printed onto
photographic paper. In the dark.
Which was a problem with the rise of the digital
age, the internet and your pressing need to upload a picture of Fluffy to the 'my cat
looks like Hitler' web forum. But while the microprocessor revolution started
to transform many parts of our lives from the 1970s onwards, the digital camera was
a relatively late invention, arriving after the video camera, the mobile phone, the laptop
computer and even the Billy Big Bass singing plastic fish. This was because the technology
that lies at the heart of it, the sensor chip, is unbelievably complicated. In a film camera, light is sent through a
lens and a shutter onto photo-sensitive film.
Which, with a subject shot in normal light,
needs just hundredths of a second of exposure to capture the image. The front end of a digital
camera works on exactly the same principle, light is focused through the lens and controlled
by a shutter and variable aperture. But instead of film, there's a light-sensitive sensor
chip that has to record all of the data in a very short space of time. There are different ways of doing this, but
we're going to concentrate on the CMOS or Complementary Metal Oxide Semiconductor sensor
that now sits in the majority of digital cameras, from those integrated in phones to fairly
chunky DSLRs (that's Digital Single Lens Reflex in the jargon) their the sort that look like
'real' cameras.
The camera's sensor is covered with tiny individual
light sensitive cells, each of which can measure the amount of light that falls on in. As the
digital camera has evolved, so have the number of these pixels on the surface of the sensor. Ten years ago, you'd struggle to get a digital
camera capable of delivering much more than a single 'megapixel' of resolution, a million
total pixels, or a grid 1200 by 900. But these days, 12 or 16 megapixels are commonplace
among top-spec 'prosumer' and professional camera.
That's enough to enable you to produce
images the size of a magazine cover with no loss in perceived quality. The cells act like the photosensitive chemicals
on old-fashioned film, reacting to the light that falls on them and then reporting to the
camera's microprocessor brain. That would be fine for the sort of moody black and white
shots favoured by gothy Instagram users. But, because most of us want to post pictures of
our lunch to Facebook in colour, it's also necessary to split the light 'seen' by the
camera into the three primary colours which can then be used to create an accurate image.
There are different ways of doing this: some
expensive cameras will even employ three different filters. But most CMOS sensors will use what's
called a 'Bayer Filter'. This is a grid of coloured filters that sit over the sensor
with red, green and blue elements over individual pixels that will only allow their respective
light colours through. Because the human eye is most sensitive to green light, which largely
determines how 'bright' an image looks, there are twice as many green pixels as red or blue.
The filters are arranged in a clever mathematical pattern, which means that the camera's brain
can interpolate using a demosaicing algorithm.
Yes, really. Or, in slightly plainer language, the camera
doesn't just look at an individual pixel on the sensor, it also looks at the pixels around
it to come up with an informed guess of what the true colour of that pixel is. Although even the most advanced sensors in
the world still struggle with the increasingly unlikely colour of Richard Hammond's hair..
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