Understanding DSLR Focus Controls

Correct focus is important because, although you can often at least partially correct for bad exposure after a shot is taken, there isn't a lot you can do when your picture is blurred beyond recognition. Point-and-shooters actually have less of a problem with correct focus (in one way) than digital SLR (dSLR) users do because the short-focal-length lenses on cheaper cameras render almost everything in sharp focus. Of course, that makes using selective focus (creatively placing parts of an image out of focus to emphasize other portions) more difficult; you win some and you lose some.

Digital SLRs give you back selective focus, while making it more critical to focus correctly. Fortunately, you have two ways to focus - manual focus and autofocus. Manual focus is accomplished by twisting a focus ring on the lens until the image looks sharp in your viewfinder. (Some cameras provide a viewfinder signal when you have manually focused correctly.) Automatic focus achieves the same results but with the camera deciding when the image is sharply focused. In both cases, the apparent contrast between your subject and its (relatively) blurry surroundings determines that focus point.

Manual focus can be difficult when you're shooting moving objects (think sports) or are in a hurry. Our brains have a poor memory for sharpness, so it takes a while for us to decide that the current setting is, in fact, sharper than the image we saw just a fraction of a second ago. The camera's autofocus mechanism has no problem comparing relative sharpness and can zero in on the right focus point very quickly - assuming that the camera actually knows what object you want to be in sharp focus in the first place.

So, manual focus can be your best choice when you want to focus on a particular object that might be difficult for the autofocus system to lock in on because, say, it's surrounded by similar-looking objects or confusing backgrounds, or you want to isolate a particular subject, like the one in Figure 1.

Automatic focus can work for you the rest of the time, particularly when you can use your dSLR's ability to choose which of several different focus zones to use. Most dSLRs have five to nine (or more) focus sensors grouped around the viewfinder screen. You can tell the camera how to use those zones or manually select a zone yourself. It's also possible to tell the camera when to lock in focus by using one of the following two common focus modes:

• Continuous autofocus: When you press the shutter release halfway, the camera sets the focus but continues to look for movement within the frame. If the camera detects motion, the lens refocuses on the new position. Use this option for action photography or other subjects that are likely to be in motion.
• Single autofocus: Press the shutter release halfway, and focus is set. It remains at that setting until you take the photo or release the button. This mode is best suited for subject matter that isn't likely to move after it has been brought into sharp focus, and for reframing a subject slightly without having the camera change the focus.
  

tech.yahoo