How Digital Cameras Work

The digital camera can be used much like a conventional film camera. Most digital cameras have more in common with automatic point-and-shoot cameras than they do with professional SLR cameras. They generally have auto-focus, and they can adjust for brightness, shutter speed and aperture automatically.

The "film" of a digital camera is a removable media-storage device (floppy disk, Flash memory card, etc.). As with a film camera, you simply replace the storage device when it's full and continue to take pictures. The difference is you don't need to develop digital pictures. You can download them directly to your computer and they are ready to use.

Understanding the Basics

Let's say you want to take a picture and e-mail it to a friend. The first step is to create a digital version of the image, so your computer can process it. There are two commonly available methods for creating a digital image:

* Take a photograph using a film emulsion, process it chemically, print it onto photographic paper and then use a digital scanner to sample the print.

* Use a device that will sample the original light that bounces off your subject to create a digital image. This device is called a digital camera. Sometimes, it is referred to as a filmless camera.

A digital camera is the easier and faster path to take.

A Filmless Camera

The key difference between a digital camera and a film-based camera is that the digital camera has no film. Instead, it has a sensor that converts light into electrical charges. All the fun and interesting features of digital cameras come as a direct result of this shift from recording an image on film to recording the image in digital form.

The image sensor employed by most digital cameras is a charge coupled device (CCD). Some low-end cameras use complementary metal oxide semiconductor (CMOS) technology. While CMOS sensors will almost certainly improve and become more popular in the future, they probably won't replace CCD sensors in higher-end digital cameras.

The CCD is a collection of tiny light-sensitive diodes, which convert photons (light) into electrons (electrical charge). These diodes are called photosites. In a nutshell, each photosite is sensitive to light -- the brighter the light that hits a single photosite, the greater the electrical charge that will accumulate at that site.

The Difference Between CCD and CMOS

As prices continue to fall, more and more people are using digital cameras. The cameras are not as common as film cameras yet, but things are certainly moving in that direction. One of the drivers behind the falling prices has been the introduction of CMOS image sensors. CMOS sensors are much less expensive to manufacture than CCD sensors.

Both CCD and CMOS image sensors start at the same point -- they have to convert light into electrons at the photosites. A simplified way to think about the sensor used in a digital camera (or camcorder) is to think of it as having a 2-D array of thousands or millions of tiny solar cells, each of which transforms the light from one small portion of the image into electrons. Both CCD and CMOS devices perform this task using a variety of technologies.

The next step is to read the value (accumulated charge) of each cell in the image. In a CCD device, the charge is actually transported across the chip and read at one corner of the array. An analog-to-digital converter turns each

pixel's value into a digital value. In most CMOS devices, there are several transistors at each pixel which amplify and move the charge using more traditional wires. The CMOS approach is more flexible because each pixel can be read individually.

CCDs use a special manufacturing process to create the ability to transport charge across the chip without distortion. This process leads to very high-quality sensors in terms of fidelity and light sensitivity. CMOS chips, on the other hand, use completely normal manufacturing processes to create the chip -- the same processes used to make most microprocessors. Because of the manufacturing differences, there are several noticeable differences between CCD and CMOS sensors.

* CCD sensors, as mentioned above, create high-quality, low-noise images. CMOS sensors, traditionally, are more susceptible to noise.

* 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 hitting the chip hit the transistors instead of the photodiode.

* CMOS sensors traditionally consume little power. Implementing a sensor in CMOS yields a low-power sensor.

Based on these differences, you can see that CCDs tend to be used in cameras that focus on high-quality images with lots of pixels and excellent light sensitivity. CMOS sensors usually have have lower quality, lower resolution and lower sensitivity. However, CMOS cameras are much less expensive and have great battery life. Over time, CMOS sensors will improve to the point where they reach near parity with CCD devices in most applications, but they are not there yet.

How the Camera Captures Color

Unfortunately, each photosite is colorblind. It only keeps track of the total intensity of the light that strikes its surface. In order to get a full colour image, most sensors use filtering to look at the light in its three primary colors. Once all three colors have been recorded, they can be added together to create the full spectrum of colors that you've grown accustomed to seeing on computer monitors and colour printers.

There are several ways of recording the three colors in a digital camera. The highest quality cameras use three separate sensors, each with a different filter over it. Light is directed to the different sensors by placing a beam splitter in the camera. Think of the light entering the camera as water flowing through a pipe. A beam splitter would be like dividing an identical amount of water into three different pipes. Each sensor gets an identical look at the image, but because of the filters, they only respond to one of the primary colors.

The advantage of this method is that the camera records each of the three colors at each pixel location. Unfortunately, cameras that use this method are both bulky and expensive.

A second method is to rotate a series of red, blue and green filters in front of a single sensor. The sensor records three separate images in rapid succession. This method also provides information on all three colors at each pixel location. But since the three images aren't taken at precisely the same moment, both the camera and the target of the photo must remain stationary for all three readings. This isn't practical for candid photography or handheld cameras.

A more economical and practical way to record the three primary colors from a single image is to permanently place a filter over each individual photosite. By breaking up the sensor into a variety of red, blue and green pixels, it is possible to get enough information in the general vicinity of each sensor to make very accurate guesses about the true colour at that location. This process of looking at the other pixels in the neighborhood of a sensor and making an educated guess is called interpolation.

The most common pattern of filters is the Bayer filter pattern. This pattern alternates a row of red and green filters with a row of blue and green filters. You may be surprised to find that the pixels are not evenly divided. In fact, there are as many green pixels as there are blue and red combined. This is because the human eye is not equally sensitive to all three colors. It's necessary to include more information from the green pixels in order to create an image that the eye will perceive as a "true colour."

The advantages of this method are that only one sensor is required and all the colour information (red, green and blue) is recorded at the same moment. That means the camera can be smaller, cheaper and useful in a wider variety of situations. In other words, it makes it possible to create an affordable handheld digital camera. The raw output from a sensor with a bayer filter is a mosaic of red, green and blue pixels of different intensity.

Digital cameras use specialized demosaicing algorithms to convert the mosaic of separate colors into an equally sized mosaic of true colors. The key is that each colored pixel can be used more than once. The true colour of a single pixel can be determined by averaging the values from the closest surrounding pixels.

Think of each photosite as a bucket or a well. Now think of the photons of light as raindrops. As the raindrops fall into the bucket, water accumulates (in reality, electrical charge accumulates). Some buckets have more water and some buckets have less water, representing brighter and darker sections of the image. Keeping to the analogy, the ADC measures the depth of the water, which is considered analog information. Then it converts that information to binary form.

Is the Number of Photosites the Same as the Number of Pixels?

If you read digital camera claims carefully, you'll notice that the number of pixels and the maximum resolution numbers don't quite compute. For example, a camera claims to be a 2.1-megapixel camera and it is capable of producing images with a resolution of 1600 X 1200. Let's do the math, a 1600 x 1200 image contains 1,920,000 pixels. But "2.1 megapixel" means there ought to be at least 2,100,000 pixels. This isn't an error from rounding off, and it isn't binary mathematical trickery. There is a real discrepancy between these two numbers. If a camera says it has 2.1 megapixels, then there really are approximately 2,100,000 photosites on the CCD.

What happens is that some of the photosites are not being used for imaging. Remember that the CCD is an analog device. It's necessary to provide some circuitry to the photosites so that the ADC can measure the amount of charge. This circuitry is dyed black so that it doesn't absorb any light and distort the image.

Output, Storage and Compression

Most digital cameras on the market today have an LCD screen, which means that you can view your picture right away. This is one of the great advantages of a digital camera: You get immediate feedback on what you capture. Once the image leaves the CCD sensor (by way of the ADC and a microprocessor), it is ready to be viewed on the LCD.

Of course, that's not the end of the story. Viewing the image on your camera would lose its charm if that's all you could do. You want to be able to load the picture into your computer or send it directly to a printer. There are several ways to store images in a camera and then transfer them to a computer.

Early generations of digital cameras had fixed storage inside the camera. To get the pictures out, they needed to be hooked up directly to a computer by cables so that the images could be transferred. Although most of today's cameras are capable of connecting to a serial, parallel, SCSI, and/or USB ports, they usually provide you with some sort of removable storage device. The main rival technologies are CompactFlash, SmartMedia and Memory Sticks, but there are others.

Compression

It takes a lot of memory to store a picture with over 1.2 million pixels. Almost all digital cameras use some sort of data compression to make the files smaller. There are two features of digital images that make compression possible. One is repetition. The other is irrelevancy.

You can imagine that throughout a given photo, certain patterns develop in the colors. For example, if a blue sky takes up 30 percent of the photograph, you can be certain that some shades of blue are going to be repeated over and over again. When compression routines take advantage of patterns that repeat, there is no loss of information and the image can be reconstructed exactly as it was recorded in the camera. Unfortunately, this doesn't reduce files any more than 50 percent, and sometimes it doesn't even come close to that level.

Irrelevancy is a trickier issue. A digital camera records more information than is easily detected by the human eye. Some compression routines take advantage of this fact to throw away some of the more meaningless data. If you need smaller files, you need to be willing to throw away more data. Most cameras offer several different levels of compression, although they may not call it that. More likely they will offer you different levels of resolution. This is the same thing. Lower resolution means more compression. And more compression means lower resolution.

Controlling the Amount of Light That Reaches the Sensor

It is important to control the amount of light that reaches the sensor. Thinking back to the water bucket analogy, if too much light hits the sensor the bucket will fill up and won't be able to hold any more. If this happens, information about the intensity of the light is being lost. Even though one photosite may be exposed to a higher intensity light than another, if both buckets are full, the camera will not register a difference between them.

Optical Zoom vs. Digital Zoom

In general terms, a zoom lens is any lens that has an adjustable focal length. Digital cameras may have an optical zoom, a digital zoom, or both.

An optical zoom actually changes the focal length of your lens. As a result, the image is magnified by the lens (sometimes called the optics, hence optical zoom). With greater magnification, the light is spread across the entire CCD sensor and all of the pixels can be used. You can think of an optical zoom as a true zoom that will improve the quality of your pictures.

A digital zoom is a computer trick that magnifies a portion of the information that hits the sensor. Let's say that you are shooting a picture with a 2X digital zoom. The camera will use half of the pixels at the center of the CCD sensor and ignore all the other pixels. Then it will use interpolation techniques to add detail to the photo. Although it may look like you are shooting a picture with twice the magnification, you can get the same results by shooting the photo without a zoom and blowing up the picture using your computer software.

Digital Camera Works - Study Questions

1. What type of conventional camera are digital cameras similar to and why?(6 Marks)

2. What are the two commonly available methods for creating a digital image? (4 Marks)

3. List the key differences between a digital and film camera.(5 Marks)

4. What are the two types of image sensors employed in a digital camera and how do they differ? (4 Marks)

5. How does the camera capture colour? (6 Marks)

6. In your own words describe the Bayer filter pattern. What are its advantages over

other methods? (5 Marks)

7. Explain why the number of photosites aren’t the same as the number of pixels.(4 Marks)

8. Describe the two methods of compression that are used.(4 Marks)

9. In your own words explain the difference between optical and digital zoom.(2 Marks)