The next most important component of a digital SLR is the lens-or, more properly, lenses- because, unlike other types of digital cameras, the lens of a dSLR is interchangeable. Galileo and Leeuwenhoek came up with improved telescope and microscope gadgets, and most of the optical breakthroughs hence have involved different kinds of shapes of glass and other materials (including non-spherical “aspheric” elements) special coatings, and clever combinations of lenses to create zooms, fish-eyes, and other innovations.

Lenses consist of precision-crafted pieces of optical glass (or plastic or ceramic material) called elements, arranged into groups that are moved together to change the magnification or focus. Lenses contain an iris-like opening called a diaphragm that can be changed in size to admit more or less light to the sensor. Lenses are mounted in a housing that keeps the elements from rattling around and provides a way to move them to adjust focus and magnification. The lens housing can include a microprocessor, a tiny motor for adjusting focus (and, in non-dSLR cameras, for zooming), and perhaps a mechanism for neutralizing camera shake (called vibration reduction). You might find a switch or two for changing from autofocus to manual focus, locking a zoom lens so it doesn’t extend accidentally while the camera is being carried, and a macro lock/lockout button to limit the seeking range of your autofocus mechanism so your lens won’t seek focus from infinity to a few inches away every time you partially depress the shutter release.

Digital photography was quickly embraced in several professional fields of endeavor where the cost of $12,000-$30,000 cameras was easily outweighed by the convenience of having digital images instantly. Here’s a quick summary:

• Photojournalism. Newspapers, news magazines, and news Web sites need visuals quickly, and can’t wait for their photographers to fly images back from Afghanistan. Digital images, which could be transmitted by phone lines in the early 1990s, can be zapped across the Internet today, sent by satellite phones, and be ready for publication immediately. Think like a photojournalist and go digital.
• Portrait photography. Digital technology has been important to professional portrait photographers because it gives them the opportunity to sell prints and enlargements immediately after the sitting, when the customers’ interest is highest. Digital portraits are easier to retouch, too. If you decide to produce portraits of your own family and friends, or need a passport photo quickly, you’ll appreciate digital technology, too.
• Photoillustration. Commercial, corporate, and industrial photographers all have taken to digital photography in a big way. For catalog work, digital photography has become the way to go because repetitive setups can be shot one after another quickly, and the finished images are immediately ready for placement in a catalog layout.
• Everything else. Although the preceding three fields were the first to adopt digital technology, today every other kind of picture taking is rapidly turning digital. Travel and vacation photography benefit from the ability to reshoot immediately if a picture is not to your liking (rather than schedule a repeat trip back to the Taj Mahal). Family and pet photography thrive on the instant feedback digital photography provides. What better way to encourage your kids to cooperate for that shooting session than to show them each group of pictures on your camera’s LCD?

All digital SLRs share a long list of common features, such as manual, aperture-priority, and shutter-priority exposure modes. All have great autofocus capabilities. Here are some of the features that vary the most from camera to camera.

■ Burst mode capabilities. For example, one Canon model grabs 4 fps for 32 JPEG images in one burst, or 11 RAW images. Another camera from the same vendor ups the ante to 5 fps, but can capture only 23 JPEG images in one blast. If you’ve got deep pockets, Canon’s top-of-the line dSLR blazes through sports photography at an 8.5 fps clip for 40 JPEG or 23 RAW images. You’ll find other burst mode capabilities in cameras from Nikon, Minolta, Olympus, Pentax, and others.

■ Anti-shake. Some dSLRs may have vibration reduction built into the camera (although, as I write this, only one model offers this capability). If you want to hand-hold your camera at low shutter speeds, or need to take rock-steady telephoto shots without a tripod, regardless of shutter speed, you’ll want to consider this capability.

■ Higher and lower ISO ratings. Some cameras offer sensitivities as low as ISO 50 and as high as ISO 6400 and beyond. Most digital SLRs have USB connections to your computer. Some have FireWire (IEEE-1394) links. We’re starting to see the ability to exchange photos between camera and computer over wireless connections, too.

■ Playback/review features. You’ll find digital SLRs with back-panel LCDs as small as 1.8 inches diagonally, and at least one new model with a 2.5-inch LCD. Using Canon as an example again, one Canon model with a 2-inch LCD offers 120,000 pixels of resolution, while another is much sharper and brighter at 230,000 pixels. how a camera’s LCD panel performs.

■ Maximum shutter speed. Some cameras top out at 1/4,000th second; others go as high as 1/16,000th second. In real life, you’ll rarely need such brief shutter speeds to freeze action. For example, if you want to use f2.8 on a bright beach or snow scene in full daylight, if your camera’s lowest ISO setting is ISO 200, you’ll probably need to use a shutter speed of 1/8,000th or 1/16,000th second.