So what is a cross-type autofocus point and how is it different from a regular focus point? Cross-type or cross points give a better chance of locking on to changes in contrast which is how AF works so your AF becomes more effective the more cross points you have. More specifically cross type sensors can detect contrast vertically and horizontally vs just just one direction for non-cross points.
Also different sensors can have different sensitivities. More sensitive ones work more precisely at wider apertures, with the trade-off being that they don’t work as well with lenses limited to slower apertures.
The central focus point is usually a cross type sensor and very often other autofocus points will be as well depending on the camera body. The Canon 5D Mark II has 9 autofocus points, but only the center focus point is cross type. The Canon 7D has 19 autofocus points and all of them are cross type. The 7D is generally better for things like sports when it come to auto focus. You can step all the way up to the yet to be released flagship model by Canon the EOS-1D X. This beast of a camera has 61 AF points, 41 points are cross-type AF sensors. Here are some definitions you should know.
- A horizontal-sensitive point or vertical-sensitive point is an autofocus point that can detect horizontal or vertical lines, respectively. These points cannot detect both at the same time.
- A high-precision sensor is an autofocus sensor that is capable of focusing within 1/3 of the depth of field of the lens, instead of simply within the depth of field. These sensors require faster maximum apertures, typically at least f/2.8, in order for them to work.
- A cross-type point is an autofocus point that can detect both horizontal and vertical lines simultaneously. Depending on the camera model, the maximum aperture may need to be at least f/2.8 or f/4 for a point to be cross-type, because the vertical-sensitive part of the AF point is a high-precision sensor.
- A dual cross-type point is an autofocus point that can detect diagonal lines as well as horizontal and vertical lines. The diagonal sensors require at least f/2.8 in order to function because they are high-precision sensors.
Basically the more autofocus point you have the better. The more of those that are cross-type autofocus point are even better yet.