Factors of Focusing AccuracyCapturing Movement
Many people have asked about the focusing accuracy of their camera/lens combination. Camera manufacturers have promised that their model can predictably focus on moving subjects such as race cars, but the camera you take home doesn’t seem able to keep up with your seven year old on the soccer field.
Speaking of focusing sensors, the most accurate sensor in any camera is the center sensor. Also, if your subject is moving fairly quickly, then turning off most of the other sensors in your camera gives your camera more processing cycles to react to your seven year old and their random movements. Some of the more advanced cameras allow you to group your focusing sensors into blocks that work together. By joining sensors, you have a bigger area looking at your child, so if one sensor sees a low contrast jersey, another in the group will see the high contrast numbers and allow the camera to continue focusing.
It is probably appropriate to discuss what other focusing options are on your camera. Virtually every camera model gives you three additional options when choosing how to use the focusing tools on your camera. The first to consider is “S” of single. What single does is maintain the original focus point as long as you keep your finger on the shutter release or another button you use to initiate auto-focus. This tool works best in two very broad situations. Situation one is where none of your focusing sensor falls conveniently on what you most want to be in focus. So, select single, put the sensor on what you do want to be sharp, press your shutter release partially to focus the camera, and when it chirps or blinks to tell you that focus has been achieved, then reframe the image to better suit your needs while keeping pressure on the shutter. When you frame the image just the way you want, then push the shutter release completely to make the exposure.
|Focus Mode Selector|
Many cameras also have a focus lock button, which will do the same thing if your shutter release is too sensitive to keep pressure without exposing an image that is not optimally framed. Situation two is when you know that something interesting is going to happen and you want to be ready. Think of a birthday party, and when you know the birthday girl is going to get something absolutely fabulous. So, lock focus on your soon to be ecstatic friend and wait for the expression you are waiting for. If you have locked in focus and exposure, then your camera doesn’t have to “think”, so the shutter will fire almost instantly. Using this technique you will get the image you see rather than the expression that occurs 2/10 of a second later.
|AF Lock Button|
Your camera also has a “C” or continuous auto-focus mode. In this mode the camera is always refocusing as you or your subject moves. Again, like most things in life, the more advanced (expensive) your camera, the quicker it will predict where your subject will be when the shutter fires. This mode is never perfect and will fail somewhat often. To improve your chances of getting most images in sharp focus, you can do several things. Put your central sensor on the subject you most care about while lightly pressing the shutter release prior to wanting images. You are giving your camera time to compute what subject you want, and how fast and in what direction it is moving. When the events happen that you expected, push the shutter release fully while keeping the main subject on the central sensor and keep it there through all the action. If you shoot thirty frames, you will have a good number of sharp action filled images. When you start and stop focusing, invariably the action that you most care about will occur when your finger is off the shutter release. Memory is cheap, images can be deleted, please do not be afraid of too many images.
Back Focusing Test
All right, you have taken to heart all of the above, you know your sport, you put your best sensor on an area of high contrast, and your images are still a little unsharp even when you have minutes to focus accurately on your subject. There could be several reasons for this. If your camera is brand new, then it is possible that it was manufactured slightly out of tolerance so that the sensor is right, but located just enough out of place that everything is just not quite accurate. Especially if your images that were shot at F11 are sharp but your images at F3.5 aren’t.