How
Your Digital Camera Works
by Greg
Scoblete
(Reprinted
with permission from DIGITAL Photographer Magazine.)
When my brother was younger, he
had a habit common among electronically inclined kids: He loved taking things
apart to see how they worked. He would spend his summers endlessly scouring
our suburban neighborhood in search of other people’s junk: discarded TVs,
radios, vacuum cleaners, any prize that he could lug (surreptitiously) into
our house and put a screwdriver and hammer to before our mother could yell at
him to get rid of that junk. His room was crammed to the ceiling with
electronic entrails – wires, plugs tubes, filters, shards of metal and glass
– you walked barefoot at your own risk. This was a more innocent time, of
course, when a high school boy could sit in his room and tinker with
electronic devices without the aura of the sinister.
My brother has since outgrown the
habit but the curiosity remains. Something tells me I shouldn’t leave him
alone with my digital camera. I can see his eyes gleam hungrily at the thought
of taking that puppy apart, piece by expensive piece, to see that makes it
tick. So instead, Mike, if you’re reading this, live vicariously.
What’s inside?
A digital camera is a fascinating
piece of electronic equipment, in part because its design has proven
challenging to manufacturers. Usually, when a product goes from analog to
digital there is a resultant increase in quality and/or performance (think
audio tape to CD or VHS to DVD) but not so with the camera. In fact, the early
incarnations of consumer-level digital cameras were worse that their analog
brothers in nearly all respects: size, picture quality, handling, not to
mention the exorbitant price tag.
Even today, as the technology
matures, wrinkles remain. You will have to spend at least two hundred more
dollars than for a film camera if you want a digital camera that produces a
photo-quality 8 X 10 inch print.
However, once you reach the
2-plus megapixel range, issues of photo quality do become a matter of
hairsplitters (you know, the type who in medieval times would argue about how
many angels could dance on the head of a pin head.) Still, examining
how a digital camera works demonstrates just why it’s not yet
perfect. Like many people, you may ponder what makes you you. The digital
camera suffers no such metaphysical quandary; indeed, the very element that
makes it digital as it is, is the charge coupled device, or CCD. It is the film
of your digital camera.
The Pick-up
A CCD is essentially a silicon
chip used to measure light. (Note: There is a competing device, the
CMOS sensor chip, which is prominent among low-resolution cameras. CMOS is
cheaper to make but offers far less image resolution. All megapixel and higher
cameras use CCD chips, so we’ll ignore the poor CMOS until it proves it can
play with the big boys.)
On the chip sits a collection of tiny
light-sensitive diodes, called photosites, or pixels, which convert
light (or, if you remember your high school physics, photons) into an
electrical charge (electrons). These pixels are light sensitive, so the
brighter the light that hits them, the greater the electrical charge that will
build on them. Conversely, the lower, less intense the light the lower the
charge on the pixel.
Light hits the CCD when you press
the camera's shutter release button. At that moment a metering cell measures
the light entering your lens (more on lenses below) and if the camera is set
on Automatic, sets the camera's aperture and shutter speed for the optimal
exposure.
When the shutter opens, each
pixel on the CCD records the light that falls on it by accumulating electrical
charges of various intensities. The CCD then ascertains the accumulated charge
of each pixel cell to form the image. The measured charges are sent across the
chip (CCDs use a special manufacturing process to create the ability to
transport a charge across the chip without distortion) and read at one corner of the array.
An analog-to-digital converter
(ADC) turns each pixel's value into a digital value (those mythic ones and
zeroes) and thus digitizes your image. But, there's more. The unfortunate
pixels on your CCD are color blind, recording only the intensity of the light
that hits them and not the color of your image. Once the ADC digitizes the
information, a microprocessor tackles the color problem in a process called
interpolation. To render a subject's color accurately, color filters must be
placed over the pixels.
The Filters
The filters are composed of the
three primary colors red, green and blue (RGB). Special attention is lavished
on green because our eyes are most sensitive to green light. Therefore, there
are as many green pixels as there are red and blue combined. Fortunately, red
and blue are not the jealous types, because if they became green with envy the
whole system would be shot.
The filters are laid down in what
is referred to as the Bayer Pattern (no relation to the aspirin, though
staring at this pattern does inspire massive migraines) which alternates a row
of red and green filters with a row of blue and green filters.
With the filters in place, each
pixel can record only the brightness of the light that matches its filter; it
lets that light through, while other colors are blocked. So, if you have a
green filter it can only recognize the green light that strikes it. A process
called interpolation allows the camera to pull together the thousands of red,
green and blue pixels and construct the final image correctly. Using the
colors of the neighboring pixels it calculates the two colors the pixel didn't
record directly.
If you remember high school art,
combining your primary colors in different measures gives you the full
spectrum of colors (including white). By combining these two interpolated
colors with the color measured by the site directly, the full color of the
pixel can be calculated and the color of your image can be rendered.
Think of this process as your
camera taking an educated guess based on what colors were recorded on the
pixel and what weren't by using surrounding pixel data.
Pixels Versus Resolution
From the above we can deduce why
resolution is measured in pixels: the more pixels on your CCD, the more
accurately light and color can be reproduced. But, here we encounter a bit of
marketing artifice. If you notice, your camera may advertise a
3.3-megapixel (3.3MP) resolution, yet the highest resolution image it
generates is 2048 x 1536 pixels. My calculator tells me that equals 3,145,728
pixels. A lot of pixels to be sure, but a good 155,000 shy of 3.3 million.
What gives?
The resolutions bandied about on
the camera box counts the number of pixels on the CCD but not the number of
pixels used by the CCD. Since the CCD is an analog device (all it does is
capture and record light's intensity), there has to be some circuitry to carry
data to the ADC (analog to digital converter) so it can do its digitizing
thing. This circuitry is placed on some of the CCD's pixels, which are then
died black so that they don't absorb any light and distort the image.
Lens With A View
Film
vs. Digital: With a film lens (above), light rays can spread out
over the film. As for a digital lens (top). light rays must hit the
film plane (chip) straight on. |
Now, the issue becomes capturing
your image (light) and aiming it at
the CCD. This responsibility falls to the lens. A lens is essentially a piece
of glass (or in the cheaper digicams, a piece of plastic) or other transparent
substance that is used to form an image of an object by focusing rays of light
from the object. It is usually circular in shape, with two polished surfaces,
either or both of which is curved and may be either convex (bulging) or
concave (depressed). The curves are almost always spherical, i.e., the radius
of curvature is constant, but higher-end cameras occasionally integrate
aspherical lenses (which are much harder to make) to achieve even greater
accuracy.
The key difference between film
camera lenses and digital camera lenses is the focal length, the distance
between the lens and the surface of the sensor. Film provides a much larger
target for light than a CCD sensor, so a digital camera lens' focal length must be
shorter to accommodate the tinier target. For perspective, a 1.3MP CCD is
about a sixth the size of a film's exposed dimension.
