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Nowadays, digital cameras have the ability to autofocus. That is all great but it is not as simple as it sounds. The autofocus in the cameras, especially digital SLRs is quite a complex system that you should endeavour to understand if you want to get the most out of it. Yes, you can point a camera and hope for the best, and that will usually work well, but then again, if you know how focussing really works, you can ensure you have more properly and consistently focused shots than by just relying on luck.
Back in the day, you’d have to drive the focus manually – spin the ring on the camera until you are in focus. It was done visually, and you were the only judge of whether your shot was in focus or not. Of course, the viewfinder was designed to make that a bit easier, but you could still get out of focus easily, especially with faster lenses. Just imagine how hard it would be tracking a race car with manual focus.
Basically, the focus drives the internal elements of the lens thereby effectively moving the field of focus forwards or backwards.
The focus won’t change if you move left or right, but it will change if you change the distance between the camera and the subject you are focused at. Therefore, if you want to focus then recompose, you’d have to do that without moving forwards or backwards at all.
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Center Point Focusing
This is the prefered focusing method for most of the photographers and it is like that for a good reason. On every camera, the central focus point is the most accurate one. Usually the central focus point is cross type, or double precision cross type. This means that instead of determining focus on one axis (regular focus point) it used two axes or four depending on the type of cross type point. The double precision one uses 4 axes to determine the focus.
Keep in mind that most modern cameras have more than one cross type point, the 7D Mark 2 has 64 of them, Canon 5D Mark 3 has 41 of them and so forth. This doesn’t mean that all of them are created equally.
The accuracy of the focus points is also determined by the quality of the lens, and the aperture of the lens. The more light the lens lets in, the more accurate the focus points can be, which is a tad obstructed by the small field of focus. However, every lens is sharpest in the middle, thus the focus point that is in the centre or near the centre will usually be more accurate than the others. This applies for phase detection focus systems too, while contrast detection focus systems rely solely on light, which determines their accuracy.