Focus Point Types
There are a lot of terms thrown around when it comes to describing the autofocus points, but the two most important are linear (also called line), and cross-type. There is a good chance that your camera has a mix of both types of sensors, and it helps to know which type is used where. For example, the Nikon D7100 has 51 autofocus points, with 15 of them being cross-type and the other 36 being linear (Figure 4.4).
Figure 4.4 Nikon D7100 has 51 AF points: 15 cross-type and 36 linear AF sensors. The sensors in green are the linear AF sensors, while those shown in orange are the cross-type. The center AF point, shown in red, is usable as a cross-type sensor when f/8.0 is the maximum aperture.
Linear AF Points
Linear AF points are single-line sensors, also called vertical-line sensors, that detect the change in the contrast along a single dimension. These are more common and less accurate type of sensors, but knowing how they work can help you get the most out of them.
The linear AF points work best when used to focus on items with detail and contrast that is perpendicular to the direction of the sensor. For example, if the sensor is placed vertically in the frame, then it works best with horizontal lines. If the sensor is horizontal, then it works best with vertical lines.
Cross-Type AF Points
Cross-type AF points are basically two linear or line sensors perpendicular to each other but covering the same area. This configuration makes them more accurate, as they can detect changes in two dimensions. Because the cross-type sensors are more accurate than the linear sensors, using them for critical focusing is a better choice.
Some high-end cameras have a double cross-type AF sensor in the center of the frame that is more accurate but only usable with a lens with a maximum aperture of f/2.8 or wider.