Sensor Size
The size of the image sensor makes a big difference in how wide an angle of view a given lens provides. Smaller sensors “see” smaller portions of the image produced by the lens (see the illustration below). The Canon EOS-1Ds Mark II and EOS 5D have full-frame image sensors, meaning they’re the same size as a full 35mm film frame (36x24mm). A given lens used on one of these cameras will have the same angle of view as when used on a 35mm SLR.

Other D-SLRs have smaller sensors. Fujifilm, Konica Minolta, Nikon and Pentax D-SLRs, for example, have APS-C-sized image sensors (about 23.7x15.6mm), with a 1.5x magnification factor—if you put a 100mm lens on one of these cameras, it frames like a 150mm lens on a 35mm SLR. So, if you specialize in wildlife and sports photography and like long lenses, a D-SLR with a smaller sensor magnifies the subject within the compositional frame. If you specialize in wide-angle work, you want a camera with a large image sensor, so it doesn’t narrow the angle of view of your wide-angle lenses; D-SLR makers and independent lens manufacturers offer super-wide-angle lenses designed specifically for the smaller-sensor D-SLRs, however, so wide-angle fans can shoot truly wide-angle shots.

The size of the image sensor also determines how many pixels the camera has, or the size of the pixels. The larger the pixels, in theory at least, the better they are at gathering light, so you get better low-light and high-ISO performance, and a wider dynamic range. And all other things being equal, the more pixels an image contains, the greater its resolution—you get images with more detail and can make larger prints (or crop into an image and blow up a small section). Of course, all other things aren’t equal. Image quality depends on many things besides pixel count, among them the sensor’s dynamic range, the camera’s A/D converter, its image-processing engine, the image compression used, the quality of the lens used for the shot, and how steadily the camera was held. Assuming the image is sharp (focused accurately, without camera shake) and properly exposed, all of today’s D-SLRs produce sufficient image quality to run a shot across two pages in this magazine or produce a superb 12x18-inch inkjet print. Resolutions range from 5 to 16.7 megapixels.

Larger image sensors have their downsides. They cost a lot more, their larger surface areas seem to attract more dust, and for telephoto fans, they reduce or eliminate the crop factor. High-megapixel images take up lots of room on memory cards (you get fewer shots per card) and require lots of computer horsepower.

Some D-SLRs use CCD (charged-coupled device) image sensors, while others use CMOS (complimentary metal oxide sensor) sensors. Long ago, CMOS sensors weren’t as good as CCD sensors, but the technology has certainly leveled the playing field, and CCD versus CMOS no longer makes a difference. Working pros use D-SLRs with CCDs and D-SLRs with CMOS imagers.

LCD Monitor
The LCD monitor allows you to view shots right after you take them, so you can make sure the exposure, sharpness and other factors are what you expected. The monitor is also used to make camera settings via a series of menu screens.

Generally, the larger the LCD monitor, the easier it is to evaluate exposed images and read histograms (exposure graphs), shooting data and menu options. But consider the pixel count as well as monitor size. More pixels mean better detail on LCD monitors, just as they do in digital photos. Check the monitor’s readability (some monitors are more legible than others, especially in bright sunlight and when viewed at an angle), and menu layout (how many buttons and arrow keys do you have to press to get to the correct menu and set something on it).

Memory Cards
Most D-SLRs take CompactFlash cards, although a few newer entry-level models use the smaller SD cards common to consumer digital cameras. Some pro models have slots for both CF and SD cards (or CF and tiny xD-Picture Cards, for Fujifilm and Olympus cameras) so you can store images simultaneously on both cards, or store RAW images on one card and JPEGs on the other.

File FOrmats
Today’s D-SLRs let you record images in the RAW or JPEG formats; some let you record in both simultaneously, which is how many prefer to do it. RAW images provide better image quality because they’re either uncompressed or losslessly compressed, and you can tailor them to your desires using RAW image software—you can change things like white balance and exposure compensation after the fact. RAW images take up a lot more space on memory cards than JPEGs do, and you have to use the RAW software to view the images and convert them into JPEGs or TIFFs.

JPEG images are compressed, so more will fit on a given memory card. At low levels of compression, image quality is little affected, but high levels can decrease image quality due to the lossy compression. Also, you can’t change the camera-applied parameters after the fact as you can with RAW images.

Color Space
All D-SLRs let you choose the color space, or the gamut of colors the images can contain. Many pro photographers use the Adobe RGB color space, which offers a wider gamut (more colors). The default sRGB color space has a smaller gamut (fewer colors), but is designed to look good on-screen and in inkjet prints, and is a good choice for those whose primary photo output is e-mailing and inkjet printing.

White Balance
With film, you have to accept the color rendition the film you’re using gives you, unless you cart along a host of expensive color-correcting filters. With digital SLRs, you can set the white balance to suit the lighting conditions: auto (which works very well in most conditions), sunlight, shade, cloudy, fluorescent, flash, etc. Some allow you to set a specific color temperature or even customize white balance for the situation at hand. Some D-SLRs even offer white-balance bracketing, in which they shoot a series of exposures, each with a different white-balance setting. The auto setting and presets do a good job in many situations, but if your work is color-critical, you might want a camera that offers color-temperature settings and custom white balance.

ISO Settings
With film, you have to shoot the whole roll at one ISO speed. With a digital SLR, you merely set the ISO to the needed speed. You can shoot every shot at a different ISO if you wish. Keep in mind, though, that as with film, higher ISOs mean images with more noise and less color accuracy, especially if underexposed or enlarged a lot, so use the higher ISOs only when necessary. When considering a digital SLR, check out its ISO range and, if possible, the image quality at the higher settings.

Subject Modes
Entry-level and some mid-range D-SLRs offer subject modes, which automatically set the camera for shooting subjects such as portraits, close-ups, landscapes, action and more. If you like these quick-access features, be aware that the pro and more advanced models don’t have them. Another thing to consider is that some cameras don’t let you apply exposure compensation in the subject modes.

Size
D-SLRs range in size from tiny (4.9x3.6x2.6 inches, 15.3 ounces) to relatively huge (6.1x6.2x3.1 inches, 43 ounces), and I’ve carted both extremes up local mountains seeking photos. It’s definitely easier to carry a Canon EOS Digital Rebel XT or Nikon D50 on a long hike than a Canon EOS-1Ds Mark II or Nikon D2x.

But size has its advantages, too. I find it easier to hold a larger camera steady with a long lens, and bigger bodies have more room for buttons, permitting more camera settings to be made without scrolling through pages of LCD monitor menus. It’s a good idea to hold each camera you’re considering and see how it feels in your hands, and how easy the controls are to operate.

Today’s D-SLRs are surprisingly sturdy. The top pro models are the most rugged, of course, but also expensive and heavy. I’ve taken several tumbles with my original mid-range D-SLR while hiking, including one in which I landed right on the camera, driving the top of its finder into the hard dirt-and-rock trail. Amazingly, the camera still works fine, and the flash unit still pops up and fires on command. Most of today’s D-SLRs are well-built, and should last a long time in normal use.

Operating Speed
Early D-SLRs took a painfully long time to start up and to wake up from the energy-saving “sleep” mode. Today’s D-SLRs are much quicker. Definitely check the start-up and wake-up time in the camera store if “decisive moments” are important in your photography. You can also check the camera specifications on the manufacturers’ Websites.

All of today’s D-SLRs have continuous shooting modes, in which the camera will take a series of images if you hold down the shutter button. How quickly they will shoot them and how many they will shoot before filling their memory buffers varies from model to model and depends on image size (they will shoot more JPEGs than RAW images because JPEGs take up less memory space) and memory-card speed. The fastest of the current D-SLR crop are the Canon EOS-1D Mark II N (8.5 fps) and the Nikon D2x and D2hs (8 fps).

Lenses & Accessories
D-SLRs made by companies that manufacture 35mm SLRs generally can use the lenses film cameras use, so if you have an AF 35mm SLR and several lenses, you can save money by buying a D-SLR from the same manufacturer. But the smaller-than-full-35mm-frame image sensors in most D-SLRs allow for smaller lenses with smaller image circles, so the D-SLR manufacturers and major independent lensmakers also offer “digital” lenses optimized for D-SLRs. Most of these digital lenses can’t be used on film cameras (or full-frame D-SLRs) because their image circles are too small—they cause vignetting.

Most D-SLRs from film-camera companies accept many of the accessories that film SLRs take. If you plan to do serious macro photography, you might want a D-SLR whose system includes extension tubes or a bellows unit. If you want to use multiple off-camera flash units with wireless TTL control, make sure you get a camera that supports it.

Unique Features
Some wonderful special features are out there—though not in the same camera. My favorites include Konica Minolta’s body-integral Anti-Shake (in the Maxxum 7D and Maxxum 5D), which turns all your lenses into anti-shake lenses; Nikon’s Lock-On (in all its D-SLRs), which keeps focus locked on the subject if it momentarily passes behind something, such as a bird flying past a tree; and Olympus’ Supersonic Wave Filter, which vibrates dust off the image sensor every time you switch on the camera. There’s also the Canon optional Data Verification Kit, which confirms that an image hasn’t been altered (handy for forensic photography), and optional wireless image-transmission systems for Canon and Nikon pro D-SLRs.

So what’s the “best” D-SLR? It’s not necessarily the one your best friend uses, or even the one your favorite famous pro uses. It’s the one that makes it easy to do what you need it to do and is in your price range.