What Are Focus Points?
Each of the autofocus (AF) points matches a sensor in the autofocus module in the camera, and each of the sensors measures the light coming in through the lens to see if it is the same on the left and right (or top and bottom). When the light is the same on both sides of the sensor, the spot that the sensor is over in the frame is in focus.
Some cameras have just a few focus points, like the older Nikon D3200, which has 11 (Figure 4.1). Other cameras have a lot, like the Canon EOS 5D Mark III, with 65 AF points (Figure 4.2), and the Nikon D810, with 51 AF points (Figure 4.3). The interesting thing to note is that although the higher-end cameras have more focus points, they don’t actually cover that much more of the frame.
Figure 4.1 The Nikon D3200 has 11 focus points, with the lower-left point active in this image.
Figure 4.2 The Canon EOS 5D Mark III has 61 focus points, giving you more options as to where you can place the focus points, but they are still clustered in the middle of the frame.
Figure 4.3 The Nikon D810 has 51 AF points spread through the center of the frame.