Nothing, ruins a photograph more than a blurry, unsharp image. One of the godsends of modern DSLR technology is the autofocus feature. But as useful as autofocus is, sometimes the camera gets it wrong and focuses on the wrong subject. Additionally, there are situations where autofocus just can’t cut it. The fantastic thing about autofocus on today’s cameras is that you can let it do all the work to get the super-sharp images. The four primary focus modes (Continuous, Single, Automatic and Manual) give you a tremendous amount of flexibility to capture exactly what you want.
Continuous Focusing Mode
AI Servo AF (Canon)/AF-C (Nikon) stands for Continuous Focus and this mode is most useful for keeping moving objects sharp within the viewfinder as you track the object. As soon as you begin to depress the shutter release, the camera goes into action and begins to focus. In Continuous focusing mode, the camera detects the subject’s movements and refocuses accordingly to keep the object sharp as a tack. This mode uses a lot of battery power because it is continuously focusing and refocusing. In addition, the autofocus technology might not accuratly predict the direction in which a chaotic, fast-moving subject is going to move… so you might still get a blur.
One Shot Focusing Mode
Next we have One-Shot AF (Canon)/AF-S (Nikon), which represent single-focus capability. In this mode, when you depress the shutter release halfway, the camera focuses on the subject just once – there’s no continuous adjustment. This mode saves battery power, and is ideal for subjects that aren’t moving. However, this mode falls short when you’re trying to capture something that’s changing positions. So unless you’re trying to get a quick shot of a deer in the early morning or hoping to immortalize Tony Romo getting tackled, then One Shot mode is probably your best bet.
Automatic Autofocus Mode
The last autofocus mode is AI Focus AF (Canon)/AF-A (Nikon), which stands for Automatic Autofocus. This is a relatively new feature which has turned out to be quite useful. In this mode the camera’s focusing computer jumps back and forth between AF-C and AF-S (Nikon)/One-Shot AF and AI Servo AF (Canon) depending on the situation. This is the default autofocus mode on cameras that have this feature. You have to remember that photography can be an art, and in art you have to go with what’s in your mind’s eye. You never know what’s going to happen next or what’s going to catch your eye, so it’s useful to have the camera make quick focus adjustments. This feature maintains focus if you change subjects or the subject moves.
Manual Focusing Mode
Manually focusing the camera is perhaps the most frustrating barrier between good and great photography. Achieving perfect focus requires using the distance measurements on the lens barrel and even perhaps measuring the distance from the lens to the subject with a tape measure; high-end photographers shoot products this way, and so do fine art photographers who are using medium format cameras. This will give you the most accurate focus point. What if you can’t take a tape measure up to a subject? Well, you have to rely on your internal sense of sharpness and know the critical focus zone that you have at the specified aperture. There is a diopter adjustment on most DSLRs (it’s right next to the viewfinder) that lets you make minute adjusts to the focusing capacity based upon any irregularities in your individual eyesight. You can also use the Depth of Field preview button to help determine focus, but this is a more advanced technique. Manual focus is important when you focus on a non-traditional subject, for example, a subject that is in the background when the foreground is busy and dominating.
Conclusion
All DSLRs allow you to turn off the autofocus and let you work with the focus ring to acquire sharp focus. Some people might find this time consuming or difficult, but as we said, photography is an art. You can use focus as means of drawing attention to or away from certain subjects. Or perhaps you want the entire frame to be out of focus to a certain degree because you want to create a dream-like quality to your image. It’s up to you. In addition, the autofocus modes have difficulty shooting through certain seemingly transparent objects that are in front of a subject, like a wire fence or quasi-reflective glass; in these cameras the autofocus mode could focus on the wire fence and not the animal behind it. In this situation, you can “outthink” the autofocus mode by depressing the shutter halfway and acquiring sharp focus on the subject in the distance.
source: http://www.exposureguide.com/focusing-modes.htm
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