2. HOW A DIGITAL CAMERA WORKS

Digital cameras are very much like the still more familiar 35mm film cameras. Both contain a lens, an aperture, and a shutter. The lens brings light from the scene into focus inside the camera so it can expose an image. The aperture is a hole that can be made smaller or larger to control the amount of light entering the camera. The shutter is a device that can be opened or closed to control the length of time the light enters.


The Nikon Coolpix 4300 looks a lot like a traditional film camera.

The big difference between traditional film cameras and digital cameras is how they capture the image. Instead of film, digital cameras use a solid-state device called an image sensor, usually a charge-couple device (CCD). On the surface of each of these fingernail-sized silicon chips is a grid containing hundreds of thousands or millions of photosensitive diodes called photosites, photoelements, or pixels. Each photosite captures a single pixel in the photograph to be.


An image sensor sits against a background enlargement of its square pixels, each capable of capturing one pixel in the final image. Courtesy of IBM.

The exposure
When you press the shutter release button of a digital camera, a metering cell measures the light coming through the lens and sets the aperture and shutter speed for the correct exposure. When the shutter opens briefly, each pixel on the image sensor records the brightness of the light that falls on it by accumulating an electrical charge. The more light that hits a pixel, the higher the charge it records. Pixels capturing light from highlights in the scene will have high charges. Those capturing light from shadows will have low charges.

When the shutter closes to end the exposure, the charge from each pixel is measured and converted into a digital number. The series of numbers can then be used to reconstruct the image by setting the color and brightness of matching pixels on the screen or printed page.



When the shutter opens, light strikes the image sensor to form the image. Courtesy of Canon.

It's all black and white after all
It may be surprising, but pixels on an image sensor can only capture brightness, not color. They record only the gray scale-a series of 256 increasingly darker tones ranging from pure white to pure black. How the camera creates a color image from the brightness recorded by each pixel is an interesting story.



The gray scale contains a range of 256 tones from pure white to pure black.


What is color?
When photography was first invented, it could only record black and white images. The search for color was a long and arduous process, and a lot of hand coloring went on in the interim (causing one photographer to comment "so you have to know how to paint after all!").


Smiling faces to greet you : mirroring contentment from within. (Title taken from label with hand-colored print.) An unidentified group of six people, two of whom are children (2 families?), standing in front of a possibly newly constructed sod house with a pitched sod roof, stovepipe, two windows and a door showing. With the people is a dog. One of the women is wearing a flat straw hat with a large ribbon. Likely taken in North Dakota.


"Fred Hultstrand copy of a photo printed from a glass plate. Glass plate borrowed from Howard O. Berg, Devils Lake, N.Dak. Brought in by Morris D. Johnson, Bismarck, N.Dak."--Back of hand-colored print. Photo likely taken by Job V. Harrison of Rock Lake, N.D. Courtesy of the Library of Congress.

One major breakthrough was James Clerk Maxwell's 1860 discovery that color photographs could be created using black and white film and red, blue, and green filters. He had the photographer Thomas Sutton photograph a tartan ribbon three times, each time with a different color filter over the lens. The three black and white images were then projected onto a screen with three different projectors, each equipped with the same color filter used to take the image being projected. When brought into register, the three images formed a full color photograph. Over a century later, image sensors work much the same way.

Colors in a photographic image are usually based on the three primary colors red, green, and blue (RGB). This is called the additive color system because when the three colors are combined or added in equal quantities, they form white. This RGB system is used whenever light is projected to form colors as it is on the display monitor (or in your eye).





RGB uses additive colors. When all three are mixed in equal amounts, they form white. When red and green overlap, the form yellow, and so on. To see how this works, visit Konica's interactive presentation by clicking the MoreInfo button below.

From black and white to color
Since daylight is made up of red, green, and blue light, placing red, green, and blue filters over individual pixels on the image sensor can create color images just as they did for Maxwell in 1860. In the popular Bayer pattern used on many image sensors, there are twice as many green filters as there are red or blue filters. That's because a human eye is more sensitive to green than it is to the other two colors so green's color accuracy is more important.


Colored filters cover each photosite on the image sensor so the photosites only capture the brightness of the light that passes through. The lenses on top of each pixel are used to collect light and make the sensor more sensitive. Courtesy of Fuji


With the filters in place, each pixel can record only the brightness of the light that matches its filter and passes through it while other colors are blocked. For example, a pixel with a red filter knows only the brightness of the red light that strikes it. To figure out what color each pixel really is, a process called interpolation uses the colors of neighboring pixels to calculate the two colors that the pixel didn't record directly. By combining these two interpolated colors with the color measured by the site directly, the full color of the pixel can be calculated. "I'm bright red and the green and blue pixels around me are also bright so that must mean I'm really a white pixel." It's like a painter creating a color by mixing varying amounts of other colors on his palette. This step is computer intensive since comparisons with as many as eight neighboring pixels is required to perform this process properly.

Here the full color of a green pixel is about to be interpolated from the eight pixels that surround it.

There's a computer in your camera
Each time you take a picture millions of calculations have to be made in just a few seconds. It's these calculations that make it possible for the camera to preview, capture, compress, filter, store, transfer, and display the image. All of these calculations are performed by a microprocessor in the camera that's similar to the one in your desktop computer.

1. WHAT IS A DIGITAL PHOTOGRAPH?

Pixels-dots are all there are
Digital photographs are made up of hundreds of thousands or millions of tiny squares called picture elements-or just pixels. Like the impressionists who painted wonderful scenes with small dabs of paint, your computer and printer can use these tiny pixels to display or print photographs. To do so, the computer divides the screen or printed page into a grid of pixels. It then uses the values stored in the digital photograph to specify the brightness and color of each pixel in this grid-a form of painting by number. Controlling, or addressing a grid of individual pixels in this way is called bit mapping and digital images are called bit-maps.

Here you see a portrait of Amelia Earhart done entirely in jelly beans. Think of each jelly bean as a pixel and it's easy to see how dots can form images.

Image size :
The quality of a digital image, whether printed or displayed on a screen, depends in part on the number of pixels used to create the image (sometimes referred to as resolution). More pixels add detail and sharpen edges.
If you enlarge any digital image enough, the pixels will begin to show-an effect called pixelization. This is not unlike traditional silver-based prints where grain begins to show when prints are enlarged past a certain point. The more pixels there are in an image, the more it can be enlarged before pixelization occurs.

The photo of the face (right) looks normal, but when the eye is enlarged too much (left) the pixels begin to show. Each pixel is a small square made up of a single color.

The size of a photograph is specified in one of two ways-by its dimensions in pixels or by the total number of pixels it contains. For example, the same image can be said to have 1800 x 1600 pixels (where "x" is pronounced "by" as in "1800 by 1600"), or to contain 2.88-million pixels (1800 multiplied by 1600).

This digital image of a Monarch butterfly chrysalis is 1800 pixels wide and 1600 pixels tall. It's said to be 1800x1600.