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Portable Digital Cameras >

Portable digital cameras frequently resemble small 35mm cameras, as shown in Figure 13.1. iDo a w m

Some models have more novel shapes, however. Rather than film, portable cameras use a o >c light-sensitive device that converts light into electrical impulses, which the camera's circuitry W—PP

can then store in some common graphics file format, such as JPEG or TIFF. You can then transfer the images to your computer via a serial or USB cable or by using a reader for the media used by the camera.

Portable digital cameras are marketed as complete or partial replacements for conventional film cameras. They frequently include the same sorts of features as film cameras, and these features should be evaluated much as you would evaluate the equivalent features in film cameras. They also possess features that are meaningless in reference to film cameras. Examples of both types of features include

• Optical quality The lens used in a camera is a large part of what determines the picture quality. There's no simple rating scale for lenses, although consumer and computer magazines sometimes perform optical tests of digital cameras.

• Zoom range Low-cost cameras usually don't include any zoom feature, but mid-range and high-end models usually have a lens that zooms. The wider the range, the more flexible the camera. On the downside, wide zoom ranges often distort images somewhat, particularly at the extreme ends of the zoom. A lens's magnification is measured by its

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focal length, which is a number expressed in millimeters (mm), as in 55mm. The size of the image you see in a final photograph depends on both the lens's focal length and the size of the film or digital imaging plate inside the camera. Because a digital camera's imaging plate need not be the same size as a frame in a roll of 35mm film, a 35mm camera's zoom range isn't directly comparable to the zoom range of a digital camera. Indeed, even two digital cameras' zoom ranges might not be directly comparable. Most digital camera manufacturers, however, advertise a 35mm equivalent zoom range, which is the zoom range of a 35mm camera that would produce the same image sizes as the digital camera does.

Figure 13.1

A portable digital camera resembles a conventional camera at first glance, but differs substantially internally.

Figure 13.1

A portable digital camera resembles a conventional camera at first glance, but differs substantially internally.

NOTE

Many digital cameras include a digital zoom feature, in which the camera crops the image and then uses interpolation features to increase its size. Digital zoom produces a blurrier picture than does optical zoom, and so should not be considered a substitute for a true optical zoom, in which the lens changes the size of the image projected on the imaging plate.

Chapter 13

Focus method Low-cost cameras use a fixed focus lens, which is permanently focused on a point that is a set distance from the camera. These cameras rely upon an optical characteristic known as depth of field to keep objects at greater and lesser distances than the fixed focus point in reasonably sharp focus. An autofocus camera, on the other hand, includes sensors that enable it to shift its focus to whatever object is in the viewfinder. Some cameras offer an override to this mode, or provide manual focus, in which you focus the camera by adjusting a control. Cameras with manual or autofocus differ in how closely they can focus upon an object.

Light sensitivity Film lightsensitivity is measured on a scale originally developed by the American Standards Association (ASA) but subsequently adopted by the International Standards Organization (ISO). Common film speeds for consumer film today are in the 100-400 ISO range, although both more- and less-sensitive films are available. Measuring a digital camera's light sensitivity is tricky because the characteristics of digital cameras are different from those of film. Nonetheless, manufacturers make the attempt. In 2000, most digital cameras have ISO equivalent ratings in the 100-200 range, although some are more sensitive than this. Most digital cameras have fixed ISO ratings, but a few can be adjusted to be more sensitive, at the cost of lower image quality.

Apertures Most cameras use a device known as an aperture to cut down the amount of light reaching the film or imaging device. Doing this is necessary in very bright light, to AVi prevent overexposure. The range of apertures varies from one camera to another, and is AR Eo measured in f-stops, as in f/4.5-f/16. The smaller the number the more light is admitted !V C

to the camera, so all other things being equal, you want an f-stop range that includes the RpU smallest value possible. TRE

Media type Most digital cameras use one of two media types: Compact Flash (CF) cards or SmartMedia cards. Both are small in size (Figure 13.2 shows a CF card next to a roll of 35mm film for scale) and can store up to tens of megabytes of data—enough for dozens or hundreds of photos at the resolutions used by most digital cameras. Both CF and SmartMedia cards are expensive, unfortunately. In early 2000, prices hovered around $2—4 per megabyte.

Resolution Digital camera resolution is often expressed in megapixels, or millions of pixels. For instance, a camera capable of recording 1280x960 images stores 1,228,800 pixels, or 1.2 megapixels. Digital cameras today range from 0.3 megapixels (640x480) to 2 megapixels or more (1600x1200 or larger). As a rule of thumb, if you expect to print your photos, you should purchase a camera capable of at least 1 megapixel resolution. Smaller resolutions might be acceptable for Web page images and other less-demanding applications.

Interface I describe interfaces in more detail later in this chapter. In general, a USB interface is preferred over a conventional serial interface, because the USB interface is faster. Using a PC card or SmartMedia adapter might be an even better way to retrieve images from your camera.

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Figure 13.2

A CF card (right) can potentially hold more than a hundred images, and yet is smaller than a 36-exposure roll of 35mm film (left).

Figure 13.2

A CF card (right) can potentially hold more than a hundred images, and yet is smaller than a 36-exposure roll of 35mm film (left).

In 2000, digital camera technology is roughly equivalent in quality to 35mm cameras for most purposes, but only if you buy at the high end of the digital range. The more affordable mid-range and low-end digital cameras are acceptable for very casual use, but you can't produce high-quality printed enlargements from the images they produce. For specific tasks, of course, digital or film could have an edge. For instance, if you need to photograph objects in low light, film is usually superior. If you need to be able to take a picture and manipulate it on a computer within seconds, digital is the only way to go.

Linux includes support for many, but not all, digital cameras. I describe the software in more detail later in this chapter, but you might want to check the camera compatibility list maintained at http://www.gphoto.org for advice on what specific models work well with Linux, and via what interfaces.

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