For A Fast AF System
What is a cross-type autofocus point and how does it help?
Most DSLR cameras come with a phase-detect autofocus sensor that is made up of several tiny points. Each of these is used to identify light patterns coming in from the lens and focus right.
These points are divided into two types, linear and cross. A linear point is a sensor in the shape of a vertical line that can recognise only a single pattern of light. If the light is in a different pattern, the lens is required to move for the linear point to recognise it. This makes autofocus very slow. On the other hand, a cross-type point that is made of two diagonal linear sensors is much faster. It is two dimensional and can recognise both the patterns of the light for fast focus.