IMAGE SENSORS

All film cameras are just dark boxes into which you can insert any kind of film you want. It's the film you choose that gives photographs distinctive colors, tones, and grain. If you think one film gives images that are too blue or red, you can change to another film. 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. The only ways to evaluate these aspects are to examine some sample photographs from the camera or read reviews written by people you trust.

Types of image sensors

Until recently, charge-coupled devices (CCDs) were the only image sensors used in digital cameras. They have been well developed through their use in astronomical telescopes, scanners, and video camcorders. However, there is a new challenger on the horizon, the CMOS image sensor that promises to eventually become the image sensor of choice in a large segment of the market. Both CCD and CMOS image sensors capture light on a grid of small pixels on their surfaces. It's how they process the image and how they are manufactured where they differ from one another.

CCD image sensor

A charge-coupled device (CCD) gets its name from the way the charges on its pixels are read after an exposure. After the exposure the charges on the first row are transferred to a place on the sensor called the read out register. From there, the signals are fed to an amplifier and then on to an analog-to-digital converter. Once the row has been read, its charges on the readout register row are deleted, the next row enters, and all of the rows above march down one row. The charges on each row are "coupled" to those on the row above so when one moves down, the next moves down to fill its old space. In this way, each row can be read-one row at a time.

CMOS image sensors

Image sensors are manufactured in factories called wafer foundries or fabs where the tiny circuits and devices are etched onto silicon chips. The biggest problem with CCDs is that there aren't enough economies of scale. They are created in foundries using specialized and expensive processes that can only be used to make other CCDs. Meanwhile, more and larger foundries across the street are using a different process called Complementary Metal Oxide Semiconductor (CMOS) to make millions of chips for computer processors and memory. CMOS is by far the most common and highest yielding chip-making process in the world. The latest CMOS processors, such as the Pentium II, contain almost 10 million active elements. Using this same process and the same equipment to manufacturer CMOS image sensors cuts costs dramatically because the fixed costs of the plant are spread over a much larger number of devices. As a result of these economies of scale, the cost of fabricating a CMOS wafer is one-third the cost of fabricating a similar wafer using a specialized CCD process. Costs are lowered even farther because CMOS image sensors can have processing circuits created on the same chip. When CCDs are used, these processing circuits must be on separate chips. Early versions of CMOS image sensors were plagued with noise problems, and used mainly in low-cost cameras. However, great advances have been made and CMOS image sensors with quality comparable to CCDs are used in some of the finest cameras.

Image sensor resolution

As you've seen, image resolution is a way of expressing how sharp or detailed images are. Low-end point and shoot cameras currently have resolutions around 3 million pixels or less, although this number constantly moves upward. Better cameras, have somewhere between 4 to 6 million pixels. The most expensive professional digital cameras give you about 12-million pixels (3000 x 4000). Although impressive, not even these resolutions match the estimated 20 million or so pixels in traditional 35 mm film and 120 million in your eye.

The Collision of Two Worlds

The term "resolution" was introduced in the computer world as a way to describe screen displays. In the early days, a screen would have a CGA or VGA resolution. Later, other names were introduced to describe even larger screens. The terms were used to describe the number of pixels on the screen. For example, a screen may have 1024 pixels across the screen and 768 down (1024 x 768). No one was concerned about the use of the term at the time it was introduced. It's only when photography became digital that another group of people entered the scene with a totally different use of the term. To photographers, or anyone in optics, resolution describes the ability of a device to resolve lines such as those found on a test chart.

As you might expect, all other things being equal, costs rise with a camera's resolution. Greater resolution also creates other problems. For example, more pixels means larger image files. Not only are larger files harder to store, they are also harder to edit, e-mail, and post on a Web site.

- Lower resolutions such as 640 x 480 are perfect 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 images.

- Higher resolutions of 3 million pixels or more, are best for printing photo-realistic enlargements larger than 5" x 7".

Kodak states that a camera with about 1-million pixels will give a 5 x 7 photo-realistic print. However, you'll get more detail and brighter colors with more pixels in the image. For prints up to 8 x 10 you can get good results with 3 million pixels. In most cases the prints are superior to those based on film. This is partly because inexpensive mass-produced prints from negatives are downright awful. Digital prints shine by comparison.

Resolution-optical and interpolated

Beware of claims about resolution for cameras and scanners because there are two kinds of resolution; optical and interpolated. The optical resolution of a camera or scanner is an absolute number because an image sensor's pixels or photoelements are physical devices that can be counted. To improve resolution in certain limited respects, the optical resolution can be increased using software. This process, called interpolated resolution, adds pixels to the image to increase the total number of pixels. To do so, software evaluates those pixels surrounding each new pixel to determine what its color should be. For example, if all of the pixels around a newly inserted pixel are red, the new pixel will be made red. What's important to keep in mind is that interpolated resolution doesn't add any new information to the image-it just adds pixels and makes the file larger. This same thing can be done in a photo-editing program such as Photoshop by resizing the image. Beware of companies that promote or emphasize their device's interpolated (or enhanced) resolution. You're getting less than you think you are. Always check for the device's optical resolution. If this isn't provided, flee the product-you're dealing with marketing people who don't have your best interests at heart.

Aspect ratios

Image sensors have different aspect ratios-the ratio of image height to width. The ratio of a square is 1:1 (equal width and height) and that of 35mm film is 1.5:1 (1.5 times wider than it is high). Most image sensors fall in between these extremes. The aspect ratio of a sensor is important because it determines the shape and proportions of the photographs you create. When an image has a different aspect ratio that the device it's displayed or printed on, it has to be cropped or resized to fit. Your choice is to loose part of the image or waste part of the paper. To imagine this better, try fitting a square image on a rectangular piece of paper.

To calculate the aspect ratio of any camera, divide the largest number in its resolution by the smallest number. For example, if a sensor has a resolution of 3000 x 2000, divide 3000 by 2000. In this case the aspect ratio is 1.5, the same as 35mm film.

Color depth

Resolution isn't the only factor governing the quality of your images. Equally important is color. When you view a natural scene, or a well done photographic color print, you are able to distinguish millions of colors. Digital images can approximate this color realism, but whether they do so on your system depends on its capabilities and its settings. The number of colors in an image is referred to its color depth, pixel-depth, or bit depth. Older PCs are stuck with displays that show only 16 or 256 colors. However, almost all newer systems can display what's called 24-bit True Color. It's called True Color because these systems display 16 million colors, about the number the human eye can distinguish.

TIP: Checking Your System

You may have to set your system to full-color, it doesn't happen automatically. To see if your Windows system supports True Color, right-click the desktop and then click Properties on the pop up menu that appears. Click the Settings tab on the dialog box and check the Color palette or Color quality setting.

Why does it take 24 bits to get 16 million colors? It's simple arithmetic. To calculate how many different colors can be captured or displayed, simply raise the number 2 to the power of the number of bits used to record or display the image. For example, 8-bits gives you 256 colors because 28=256. Here's a table to show you some other possibilities.

Some digital cameras (and scanners) use 30 or more bits per pixel and professional applications often require 36-bit color depth, a level achieved only by professional-level digital cameras. These extra bits aren't used to generate colors that are later displayed. They are used to improve the color in the image as it is processed down to its 24-bit final form and then discarded.

Sensitivity

An ISO (International Organization for Standardization) number that appears on the film package specifies the speed, or sensitivity, of a silver-based film. The higher the number the "faster" or more sensitive the film is to light. If you've purchased film, you're already familiar with speeds such as 100, 200, or 400. Each doubling of the ISO number indicates a doubling in film speed so each of these films is twice as fast as the next fastest.

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. To get more light you need a longer exposure time that can lead to blurred images or a wider aperture that gives you less depth of field. All things being equal, it's better to get an image sensor with a higher ISO because it will enhance freezing motion and shooting in low light. Typically, ISOs range from 100 (fairly slow) to 3200 or higher (very fast).

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 (increasing its gain). Some cameras even increase the gain automatically. This not only increases the sensor's sensitivity, it also increases the noise or "grain," making the images softer and less sharp.

Image quality

The size of an image file depends in part on the resolution of the image. The higher the resolution, the more pixels there are to store so the larger the image file becomes. To make large image files smaller and more manageable most cameras store images in a format called JPEG after its developer, the Joint Photographic Experts Group and pronounced "jay-peg." This file format not only compresses images, it also allows you to specify how much they are compressed. This is a useful feature because there is a trade-off between compression and image quality. Less compression gives you better images so you can make larger prints, but you can't store as many images. More compression lets you store more images and makes the images better for posting on a Web page or sending as an e-mail attachment. The only problem is that your prints won't be as good.

Instead of using compression, some cameras allow you to change resolution as a way of controlling the size of image files. Because you can squeeze more 640 x 480 images into a storage device than you can squeeze 1024 x 768 images, there may be times when you'll want to switch to a lower resolution and sacrifice quality for quantity.

Frame rate

Henri Cartier-Bresson is famous for his photographs that capture that "decisive moment" when random actions intersect in a single instant that makes an arresting photograph. His eye-hand coordination was unrivaled, and he was able to get the results he did because he was always ready. There was never any fumbling with controls or lost opportunities. Most digital cameras have an automatic exposure system that frees you from the worry about controls. However, these cameras have other problems that make decisive moments hard to capture. There are two delays built into digital cameras that affect your ability to respond to fast action when taking pictures.

- The first brief delay you experience is between pressing the shutter button and actually capturing the image. This delay, called the refresh rate, occurs because the camera clears the image sensor, sets white balance to correct for color, sets the exposure, and focuses the image. Finally it fires the flash (if it's needed) and takes the picture.

- The second delay, the recycle time, occurs when the captured image is processed and stored. This delay can range from a few seconds to half a minute.

Both of these delays affect how quickly a series of photos can be taken one after another, called the frame rate, shot-to-shot rate, or click to click rate. If the delays are too long, you may miss a picture. To capture rapidly unfolding actions, many cameras have a burst, continuous, or sequential mode that lets you take one photo after another as long as you hold down the shutter button. To make this possible, these cameras store the images in a memory area called a buffer and then process them when the sequence is finished. How many pictures you can take at one time depends on the size of the images and the size of the buffer.