Introduction
Digital cameras and traditional cameras are very much alike in many respects. The biggest difference is the way images are captured. With traditional films, they are caprutured on silver-based film. With digital cameras, they are captured on solid-state devices called image sensors.

DIGITAL PHOTOGRAPH
This class is about digital cameras and the photographs they capture. Understanding the end product, the digital photograph, is a good place to begin understanding the entire digital photography process.

Pixels—Dots
Digital photographs are made up 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 containing hundreds of thousands or millions of of pixels. The computer or printer then uses the values stored in the digital photograph's file 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.

Image Size and Number of Pixels
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 to an image, sharpen edges, and increase resolution.
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.

Describing Image Sizes
The size of a digital 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 1600 * 1200 pixels (where "*" is pronounced "by" as in "1600 by 1200"), or to contain 1.92 million pixels (1600 multiplied by 1200).

DIGITAL FILM and THE IMAGE SENSOR
Unlike traditional cameras that use film to store an image, digital cameras use a solid-state device called an image sensor. These fingernail-sized silicon chips now contain millions of photosensitive diodes called photosites. Each of these photosites records the intensity or brightness of the light that falls on it. Each photosite reacts to the light that falls on it by accumulating a charge; the more light, the higher the charge. The brightness recorded by each photosite is then stored  as a set of numbers that can then be used to set the color and brightness of dots on the screen or ink on the printed page to reconstruct the image.

Color Depth
Color depth refers to how many bits are used to record each color. The more bits used, the richer the colors will appear. Most affordable cameras offer 24-bit color depths (8 bits for Read, 8 for Green, and 8 for Blue), although 30- 36 bit models exist. Professional applications often require 36-bit color depth, a level achieved only by professional-level digital cameras. The more bits that are assigned to each color, the more gradations can be stored.

For example,
8 bits stores = 256 shades
10 bits stores = 1024 shades
12 bits stores = 4096 shades

Combining the three colors captured in 8, 10, or 12 bits gives you a final full-color image is 24, 30, or 36 bit color.

Image Size
Most consumer level cameras have pixel counts of about 2 million pixels. From these cameras, you can make good quality prints up to about 8 x 10 inches. Lower resolutions are fine for Web publishing, e-mail attachments, small prints, or images in documents and presentations. For these uses, higher resolutions just increase file sizes without significantly improving the image.

Aspect Ratios
They even have different aspect ratios—the ratio of image height to width. The ratio of a square is 1:1 and that of 35mm film is 1.5:1. Most image sensors fall in between these extremes. Some cameras have one aspect ratio for the image sensor and another for the viewfinder. This means you don't see the entire scene that will be captured when you take a picture.

To calculate the aspect ratio of any image sensor, divide the largest number in its resolution by the smallest number. For example, if a sensor has a resolution of 1800 x 1600, divide 1800 by 1600. In this case the aspect ratio is 1.33, different from 35mm film.

Image	Width x Height	Aspect Ratio
35 mm film	36 x 24 mm	1.50
Display monitor	1024 x 768 pixels 800 x 600 640 x 480	1.33
Nikon 950	1600 x 1200 pixels	1.33
Photo paper	4 x 6 inches	1.50
Photo paper	8 x 10 inches	1.29
HDTV	16 x 9	1.80
Stationary	8 1/2 x 11	1.29

Sensitivity
The speed, or sensitivity, of a film is given as an ISO (International Organization for Standardization) number that appears on the film package such as 100, 200, or 400. The higher the number the "faster" or more sensitive the film is to light.

Image sensors are also rated using equivalent ISO numbers. Just as with film, an image sensor with a lower ISO needs more light for a good exposure than one with a higher ISO. Higher ISOs enhance freezing motion and shooting in low-light.

Some cameras have more than one ISO rating. In low-light situations, you can increase the sensor's ISO by amplifying the image sensor's signal more (increasing its gain). Some cameras even increase the gain automatically. This not only increases the sensor's sensitivity, it also increases the noise; the equivalent of grain in a traditional silver-based image.

Image Quality
The image sensor in cameras has an impact on the quality of images. In film cameras it's the film you choose that gives photographs distinctive colors and tones.With digital cameras, the "film" is permanently part of the camera so buying a digital camera is in part like selecting a film to use. Like film, different image sensors render colors differently, have different amounts of "grain," different sensitivities to light, and so on.