As with exposure, digital cameras use different kinds of methods to collect focus information and then evaluate it to produce the correct focus. The image will be in sharp focus at the position of highest contrast. An autofocus system may rely on the ambient illumination on the subject, or use a special autofocus light source built into the camera to improve the lighting under dim conditions. of the pattern.

Autofocus Considerations

I outlined a few of them in the section on manual focus.

• Autofocus speed. The speed at which your autofocus mechanism operates can be critical. Because autofocus is built into the lens, rather than the camera, this speed is highly dependent on the design of the lens. Some models focus more slowly than others, either because focusing involves lens elements that can’t be shifted rapidly, or because the motors and actuators in the system move too darn slowly.
• Autofocus technology. Different digital SLRs use different autofocus systems, even among systems offered by the same vendor. These differences can involve the type and number of sensors used to calculate focus. Sensors can consist of lines of pixels that are evaluated, or cross-hatched sections that cover more area. Different sensors may be used in bright light than in dim light. Plus, you may be able to move the sensor’s viewpoint from place to place in the frame, using your camera’s cursor keys.
• Autofocus evaluation. How and when your camera applies the autofocus information it calculates can affect how well your camera responds to changing focusing situations. As with exposure metering systems, your camera may use the focus data from the various sensors differently, depending on other factors and settings. You use the equivalent of spot focus, center-weighted focus, and something that operates very much like averaging focus (although the camera will still select a point of interest to zero in on, no matter where it appears in the frame).

With manual focus, you twist the focus ring on your lens until the image you want pops into sharp focus. Many dSLRs have a cool feature: Even if you’re focusing manually, a focus indicator LED may light up when you’ve achieved correct focus, giving you some helpful extra confirmation. Some points to consider about manual focus are these:

• Speed. Manual focus takes more time, compared to the speedy operation of autofocus systems. If you’re shooting contemplative works of art, portraits in which your subjects will generally stay put for a period of time, or close-up pictures, the speed of manual focus may be no consideration at all. For action photography, however, you may not be able to change focus quickly enough to keep up.
• Non-Memory Effect The eye (the brain actually) doesn’t remember focus very well. That’s why you must jiggle the focus ring back and forth a few times using smaller and smaller movements until you’re certain the image is sharply focused. You never really know if optimal focus is achieved until the lens starts to de-focus. So, manual focus may actually be a trial-and-error experience.
• Difficulty. Focusing is most easily done when the image is bright and clear, the depth-offield of the image being viewed is shallow, and there is sufficient contrast in the image to make out details that can be brought in and out of focus. Unfortunately, you’ll frequently encounter scenes that are dim and murky, with little contrast, and be using a slow lens that compounds the problem.
• Accuracy. If you’re using selective focus creatively, manual focus is the only way to go.
• Following Action. Today’s autofocus systems are sophisticated enough that they can use predictive focus to track moving subjects and keep them in focus as they traverse the frame.

Because most digital SLR photographers were already seasoned veterans before they began using a dSLR, they already have a considerable advantage over neophyte photographers who must master digital technology at the same time they are learning photographic basics. You know to hold the camera steady in dim light and how to make a background less prominent by throwing it out of focus.

You understand terms like lens flare, motion blur, and grain, and may have more than an inkling about things like solarization, halftones, mezzotints, or unsharp masking.

• Basic composition. Seasoned photographers know how to line up shots to produce a pleasing composition. You’ll find this skill valuable with dSLRs, because their WYSIWYG viewpoint makes composition more precise.
• Choosing lenses. Beginners don’t choose lenses or zoom settings. Photographers understand that lens choice is an important part of the creative process to, say, compress the apparent distance between objects, emphasize the foreground, or produce pleasing portraits.
• Using selective focus. Point-and-shoot cameras generally don’t offer much flexibility in applying depth-of-field.
• Choosing a film “look.” If you’re a veteran film photographer, you’re used to choosing one film because it provides vivid, saturated colors even on overcast days, or another film because it has accurate flesh tones for portraits, or a third because it has extra contrast that makes product shots look their best. You can apply this knowledge to your digital camera to select saturation, contrast, and exposure settings that suit the exact look you want.
• Knowledge of what you can do in the film and digital darkroom. Experienced photographers know how and when to take advantage of image-editing techniques, such as retouching, compositing, color correction, and special effects. These can be used to fix problem images, or make a good image a great one.