There are several focus points that you often see via the viewfinder in modern DSLR cameras. They are called AF points (autofocus points). The number of these autofocus points varies for different DSLR models with the range from 5 to 52 autofocus points.
The camera needs these Autofocus points to make a focus on a certain subject. When you hold down the shutter button halfway, you will likely see these points first. In many cameras, uou’ll often hear a “beep” sound when the AF points appear in the camera viewfinder (often shown in the green or red color). When you use your camera, if the AF points are automatically selected, you will see the AF points light up indicating that the camera is focusing on those points.
The automatic autofocus setting can be very nice, if you set a large depth of field and the targeted subjects are not moving. But sometimes, for some certain kinds of subjects, the focusing point may not be clear enough for the camera autofocus. For instance, when you are attempting to capture an image of a bird on a highly contrasted background, the autofocus may aim at the more highlighted background at the back. Therefore, sometimes you should use the manual AF setting to achieve the better results.
Manual AF Setting
Manual AF setting refers to the way in which only one autofocus point is selected, which brings you an accurate position on the focused point. With some high-end models, for example the Canon EOS 7D, the AF mechanism is very smart, in which you will be able to not only select the single point but it is also possible to select a group or area of the photo to focus on. Thanks to these advanced AF systems, it helps reduce the chances of photographers to encounter the wrong focus issue.
A Large Number of AF Points Is Often Useful
A large number of AF points do help in case you shoot many action photos or if you photograph pets or kids which are often moving. The larger number of AF points is very useful for reduce the opportunities that the subject move out of a focusing point. If portraits or landscapes are what you often shoot, the number of AF points does not need to be large because you can change the subject or your location easily.