How Many Pixels Are Enough? A Guide to Choosing a Digital Camera
- Jun 21, 2002
The first thing people want to know about a digital camera is "how many pixels does it have?" Although this is not a bad question to ask, it's not the only factor to consider when choosing a camera. Cameras with higher pixel counts generally create higher quality pictures, but they also create larger files that aren't appropriate for some uses.
For example, if you're purchasing a camera to use primarily for sending snapshots via e-mail or the Web, you have to resize your images to a smaller size to reduce upload and download times. You don't need a 3-megapixel camera if you'll always resize the images down to a megapixel or less.
To help you decide how many pixels you need (and how much money you need to spend to get those pixels), the following topics are covered in this chapter:
- Are more pixels always better?
- How film and digital cameras work
- Where do pixels come from?
- How image sensors work
- What's coming down the road
If you plan to print most of the picture you take with your digital camera, you want as many pixels as you can get. This is especially true if you'll be printing your images on a high-quality photo printer. The larger the print, the more pixels you need to get an acceptable picture. Table 3.1 shows how many pixels you need for several popular print sizes.
Table 3.1Approximate Number of Pixels Needed to Produce a High-Quality Print at Different Paper Sizes
Maximumm Print Size
4 x 6''
5 x 7''
8 x 10''
11 x 14''
Let Me Count the Pixels...
Many people-including camera manufacturers-often incorrectly use the term resolution to refer to the pixel count, or the number of pixels produced by a camera. Resolution refers to the camera's ability to capture small details. Pixel count is simply the number of pixels produced by the camera's image sensor. Although the two terms are related, they're not the same.
Is More Always Better?
The short answer is yes. All things being equal, a camera with more pixels produces better pictures than a camera with fewer pixels. The pixel count determines the overall size and quality of the images created by a digital camera. In general, the more pixels, the more detail a picture contains. Pictures with more details appear sharper than pictures with less detail.
Pass the Dots, Please
Printer technology is improving almost as fast as camera technology. As discussed in Chapter 9, "Inkjet Printers," inkjet printers (the most popular type of printer for printing digital camera images) print images using tiny dots of ink. The more dots the printer can produce, the clearer the image appears.
Just a few years ago, 600 dot per inch (DPI) printers were the norm. Today, there are several inexpensive printers on the market that can print more than 2,000 DPI. These new printers take better advantage of the increased pixel counts and higher detail of newer digital cameras.
In a film camera, picture quality is a factor of the size of the film, the sharpness of the lens, and the resolving power of the film. In a digital camera, resolution is determined by the number of pixels in the image sensor, the sharpness of the lens, and the camera's ability to convert raw pixels into an electronic image. In the film world, the easiest way to get more detail is to use a larger piece of film. In the digital world, you get more detail by creating more pixels-up to a point, as you'll see in a moment.
Pixel count has become the main yardstick used to measure and compare cameras. Digital cameras produce images with millions of pixels, so the term megapixel is used as shorthand for "a million pixels."
To understand what pixels are and why they're important, it helps to understand how conventional film cameras work.
How Film Works
Photographic negative film contains millions of tiny, light-sensitive silver halide crystals on the surface of the film. Each individual picture on a roll of film is recorded on a unique area on the film called a frame. As you take pictures and wind the film, the most recently exposed frame moves out of the area behind the camera's lens, and another, unexposed frame moves into place, until you get to the end of the roll of film.
When the film is developed, the crystals that were exposed to light remain on the film; those that weren't exposed to light are removed in the developing process. (The process works just the opposite for slide film, which produces a positive image instead of a negative.) As a result, dark areas on the film have more crystals; lighter areas have fewer.
Where Do Pixels Come From?
Digital images on your computer screen are composed of a series of colored squares called pixels. Each pixel is described by three or four numbers that define each pixel's color and brightness. In the RGB color space system most commonly used for consumer digital imaging, each picture has a red, green, and blue value, and each value ranges from zero (dark) to 255 (bright). Red, green, and blue light combine to make white, so a pixel with an RGB value of 255,255,255 displays as 100% white. Similarly, a pixel with a value of 0,0,0 displays as black, and a pixel with a value of 0,255,0 displays as bright green. There are other color space systems besides RGB. For example, the cyan, magenta, yellow, black (CMYK) system is often used for images that are to be printed via conventional four-color offset printing presses, which use cyan, magenta, yellow, and black inks.
What's a JPEG, Anyway?
Digital images are stored in electronic files, and the most common of these is the Joint Photographic Experts Group, or JPEG, format. JPEG files can be stored with varying degrees of electronic compression, which make the files smaller and faster to work with. Information about file formats and compression is presented in more detail in Chapter 20, "Outsourcing Your Printing."
Digital cameras are basically small computers that convert live images into digital files. They record images by electronically detecting light (photons) striking the face of an electronic image sensor. The face of the image sensor contains millions of light-sensitive transistors called phototransistors or photosites. Each photosite represents one pixel, and the terms are often used interchangeably when discussing image sensors. When light strikes one of the photosites, it causes a change in the electrical charge flowing through the transistor. The stronger the light, the stronger the change.
The camera builds an image from the array of pixels by electronically scanning the contents of each pixel. Image sensors are monochrome; that is, they see light as black or white. To make a black-and-white sensor see color, each photosite on the sensor is covered with a layer of color filters called a color filter array, or CFA. Most cameras use red, green, and blue (called GRGB) CFAs, although some use a cyan, yellow, green, and magenta (CYGM) array. For clarity, I'll illustrate the more common GRGB arrangement, but the process is the same for CYGM sensors.
The dye layers effectively make each photosite sensitive to only a single color, depending on the color of the dye. The dye is applied in a pattern (called a Bayer pattern) such that each row has either alternating red and green or blue and green pixels. If you do the math, you'll see that in a GRGB sensor, there are twice as many green pixels are there are red or blue. That's because green provides much of the perceived detail in the picture, while red and blue contribute relatively little detail information. By using twice as many green pixels, camera designers can squeeze the most detail out of the image sensor.
When you take a picture, a chip inside the camera called an image processor reads the data collected by the image sensor. The processor mathematically combines the data from each pixel with the data from its neighboring pixels to produce an RGB value for each pixel. The RGB data is collected and saved as an image file on the cameras' storage media.