Get the facts before you buy: 10 secrets of digital SLRs that you absolutely,
positively have to know now.
5) With Autofocus, More Is More.
The principles of autofocus are pretty simple: Electronics in the viewfinder system detect scenic detail and signal the lens to move one way or the other to sharpen it. Along the way, though, AF seems to have gotten awfully complicated -- just look at any DSLR manual. Here are your choices.
Linear versus cross sensors: AF is designed to detect lines. Linear sensors detect lines oriented only one way -- horizontally or vertically. Cross sensors detect lines in either direction. Cross sensors are better than linear, and, although most DSLRs now have at least one in the center, the more the better.
Single versus continuous mode: Single stays locked on until you take the picture or let up on the shutter button -- it won't let you take a picture if it hasn't focused. Continuous keeps refocusing and lets you shoot even if it's not focused. Single AF is best for most situations. Use continuous for jumpy action.
Tracking versus dynamic focusing: AF can predict where a moving subject will be when you shoot. Tracking focus follows action toward or away from the camera. Most DSLRs do this in continuous AF. Dynamic focusing can follow a subject across the frame -- DSLRs with multiple AF points across the frame can do this.
That's it. Really. Camera makers devise all sorts of variations (like "Dynamic Area AF With Closest Subject Priority") that you shouldn't lose sleep over. Confused about what assortment of modes to use? For most picture taking, the simplest AF setup -- single-frame, picking your focus point manually -- works fine. For breakneck action, turn on every bell and whistle: continuous AF, dynamic tracking, automatic focus point selection.
Pop Photo TIP: The important things to look for are speed and sensitivity -- how quickly the AF catches a detail, and how dim the light can be for it to work. We test speed and sensitivity at various light levels (EV), and report our findings in a graph. Action shooters want speedy AF at mid-EV levels; available-light enthusiasts should look for fast performance at low EVs.
6) The Meter Is the Smartest -- and Stupidest -- Thing in Your DSLR.
Digital photography is much like shooting slide film in that it requires precise exposure, especially if you capture and store JPEG files instead of RAW (see #9). This is where your camera's TTL (through-the-lens) metering comes in -- it measures light and provides an exposure setting that will give you a midtone in a photograph. But the meter doesn't know how to do anything else.
A common form roughly averages all the light coming through the lens and gives a midtone reading for that. Because the sensitivity is concentrated in the center of the frame, this is called centerweighted metering. DSLRs also let you meter a small area -- called spotmetering.
Then there's evaluative metering, which makes an educated guess about where to place the reading. This is where metering systems get truly smart. Evaluative meters divide the frame up into a number of segments and compare the readings in each. Onboard software analyzes these readings and chooses a pattern appropriate to that kind of lighting -- for example, concentrating the reading on the shadow area of a backlit subject. Some systems, like the Matrix metering in Nikon's D200 and new D80 cameras, even have databases of sample scenes with which to compare the readings.
Every DSLR on the market comes with all three metering types. But there are variations: Some cameras let you link the spotmeter area with any autofocusing zone in the frame -- handy for off-center subjects, but not essential. Nikon DSLRs allow you to adjust the size of the centerweighted meter's central zone. The Canon EOS 30D and 5D give you a "fat spot," as well as a true narrow-angle spotmeter.
Your meter can still be tricked by complicated lighting situations. That's why exposure compensation lets you nudge any reading up or down for lighter or darker pictures.
Pop Photo Tip: Use evaluative metering. It's reliable in a wide variety of shooting situations and takes a ton of the fuss out of metering. This doesn't mean you shouldn't check your exposures, especially in important pictures. Familiarize yourself with the exposure compensation procedure so you can adjust exposures quickly. And if you want to make spotmetering easy, learn to use the autoexposure lock, which lets you set an exposure with the press of a button.