Diagnosing Digital Photography Focus Problems - dummies

Diagnosing Digital Photography Focus Problems

By Julie Adair King

Several factors affect focus in digital photography, so to find the solution to a focusing concern, you first have to identify the cause. Here’s a quick symptoms list that will point you in the right direction:

  • The entire picture is blurry. This problem, illustrated in the first photo below, is the result of camera shake — camera movement during the exposure. Camera shake is a possibility any time you handhold the camera, and the slower the shutter speed, the greater the risk. To remedy the situation, put the camera on a tripod or find another surface on which to place the camera so that you can get a steady shot.
    camera shake
    Camera shake causes allover blur (left); use a tripod to ensure sharp shots (right).

    If you must handhold the camera, find out whether your camera or lens offers image stabilization. That feature, when enabled, can compensate for small amounts of camera shake.
    The subject is blurry, but the rest of the picture is sharp. This problem can be caused by a couple of errors:

    • The subject moved, and the shutter speed wasn’t fast enough to freeze the action. To catch a tack-sharp picture of a moving subject, you need a fast shutter speed. How fast depends on your subject’s speed. In the first photo below, for example, the duck on the left was moving his head too fast to be captured cleanly at a shutter speed of 1/60 second. Bumping the shutter speed up to 1/250 second froze the action successfully, as shown in the photo on the right.

      In order to maintain the proper exposure when you raise the shutter speed, which reduces exposure time, you must choose a lower f-stop setting (to allow more light into the camera) or raise the ISO setting (to make the camera more sensitive to light). In the image above, the ISO was adjusted.

    • The subject is outside the focusing zone. By default, most cameras base focus either on the object closest to the lens or the object at the center of the frame. To make things even more complex, the designated focus point may change depending on your exposure mode — the camera may use a different point in Auto mode, for example, than it does in Close-up mode.

      faster shutter speed
      If moving objects are blurry, try using a faster shutter speed.

      Obviously, it’s critical to know the spot your camera targets when setting focus so that you can be sure that your subject is within that focusing area. Depending on your camera, you may be able to designate a specific focusing point or area other than the one used by default.

  • Too much (or too little) of the scene is in focus. When you focus the camera, you establish the point of sharpest focus. Whether objects in front of or behind the focus target are also in focus depends on the depth of field. Your camera’s focus controls don’t affect this aspect of your photos; instead, you manipulate depth of field by adjusting aperture (f-stop), focal length, and camera-to-subject distance.

  • The camera won’t focus. Assuming that your camera isn’t broken, you may simply be too close to your subject. Again, every lens has a close-focusing limit, which you should be able to determine with a look at your camera or lens manual.

If you’re using autofocusing, another possible cause is the subject itself. Even the best autofocusing systems have trouble with certain subjects, such as reflective surfaces and objects under water or behind a fence. Dim lighting is also problematic for autofocusing systems, as are scenes in which little contrast exists between your desired focus point and the surrounding area. The easiest fix is to focus manually, if your camera permits it.

Another option is to look for an easy-to-focus object that’s the same distance from the camera as your subject, lock focus on that object, and then reframe to your desired composition and take the picture. Again, check out “Getting good autofocusing results” for specifics on that technique.

  • The scene in the viewfinder looks blurry even when the camera indicates that focus is set. Go ahead and take the picture, and then determine whether the image is in fact correctly focused. If it is, you may simply need to adjust the viewfinder to your eyesight. You do this by using a diopter adjustment control, a little dial or switch located near the viewfinder. On the other hand, if your test shot is as blurry as the viewfinder indicates and none of the preceding solutions solves the problem, your camera or lens may need repair.

Dirt or dust on your lens can also cause focusing miscues, so keep your lens-cleaning supplies handy. If you have a filter attached to the lens (such as a neutral density filter), remove it to make sure that no debris is trapped between it and the lens. Then clean both the lens and the filter.