20 Sep 08 Digital Camera Basics-images
Author: Brian Lee
In the past twenty years, most of the major technological breakthroughs in consumer electronics have been built around the same basic process: converting conventional analog information (represented by a fluctuating wave) into digital information (binary information represented by ones and zeros, or bits). This fundamental shift in technology has changed how we handle visual and audio information — it completely redefined what is possible.
The digital camera is one of the most notable examples of this shift because it is so truly different from its predecessor. Conventional film cameras depend entirely on chemical and mechanical processes — you don’t need any electricity whatsoever to operate them, other than for a flash. On the other hand, all digital cameras have a built-in computer, and all of them record images electronically.
The new approach has been enormously successful. Since film usually provides better picture quality, digital cameras have not completely replaced conventional cameras. But, as digital imaging technology has improved, and prices dramatically decreased, digital cameras have rapidly become more popular.
In this article, we’ll find out exactly what’s going on inside these amazing digital-age devices.
Understanding the Basics
Let’s say you want to take a picture and e-mail it to a friend. To do this, you need the image to be represented in the language that computers recognize — bits and bytes, or binary information. Essentially, a digital image is just a long string of 1s and 0s that represent all the tiny colored dots — or pixels — that collectively make up the image. If you want to get a picture into this form, you have two options:
1) You can take a photograph using a conventional film camera, take the film to a developing lab that processes the film chemically, prints it onto photographic paper, and then place the picture on a digital scanner to sample the print (record the pattern of light as a series of pixel values).
2) You can directly sample the original light that bounces off your subject, immediately breaking that light pattern down into a series of pixel values — in other words, you can use a digital camera.
At its most basic level, this is all there is to a digital camera. Just like a conventional film camera, it has a series of lenses that focus light to create an image of a scene. But instead of focusing this light onto a piece of film, it focuses it onto a semiconductor device that records light electronically. A computer then breaks this electronic information down into digital data. All the fun and interesting features of digital cameras come as a direct result of this process.
Instead of film, a digital camera has a sensor that converts light into electrical charges.
The image sensor employed by most digital cameras is a charge coupled device (CCD). Some cameras use complementary metal oxide semiconductor (CMOS) technology instead. Both CCD and CMOS image sensors convert light into electrons. Without getting too technical, a simplified way to think about these sensors is to think of a 2-dimentional array of thousands or millions of tiny solar cells.
Once the sensor converts the light into electrons, it reads the value (accumulated charge) of each cell in the image. This is where the differences between the two main sensor types become a factor:
A CCD transports the charge across the chip and reads it at one corner of the array. An analog-to-digital converter (ADC) then turns each pixel’s value into a digital value by measuring the amount of charge at each photosite and converting that measurement to binary form. CCD sensors create high-quality, low-noise images. CCD sensors have been mass produced for a longer period of time, so they are more mature. They tend to have higher quality pixels, and more of them.
CMOS devices use several transistors at each pixel to amplify and move the charge using ordinary wires. The CMOS signal is digital, so it needs no ADC. Because each pixel on a CMOS sensor has several transistors located next to it, the light sensitivity of a CMOS chip is lower (many of the photons hit the transistors instead of the photodiode.) CMOS sensors traditionally consume little power. CCDs, on the other hand, use a process that consumes lots of power.
The amount of detail that the camera can capture is called the resolution, and it is measured in pixels. The more pixels a camera has, the more detail it can capture and the larger pictures can be without becoming blurry or “grainy.” High-end consumer cameras can capture over 12 million pixels. Some professional cameras support over 16 million pixels, or 20 million pixels for large-format cameras. For comparison, Hewlett Packard estimates that the quality of 35mm film is about 20 million pixels.
Exposure and Focus
Just as with film, a digital camera has to control the amount of light that reaches the sensor. The two components it uses to do this, the aperture and shutter speed, are also present on conventional cameras.
Aperture: The size of the opening in the camera. The aperture is automatic in most digital cameras, but some allow manual adjustment to give professionals and hobbyists more control over the final image.
Shutter speed: The amount of time that light can pass through the aperture. Unlike film, the light sensor in a digital camera can be reset electronically, so digital cameras have a digital shutter rather than a mechanical shutter.
These two aspects work together to capture the amount of light needed to make a good image. In photographic terms, they set the exposure of the sensor.
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