How to utilize crosstype autofocus points
Choose the cross-type AF point(s) for more reliable AF
It isn’t just the number of AF points – and the speed at which you can select them – that makes a difference to the autofocus accuracy, it’s their precision and sensitivity too.
Not all AF points are made equal. Standard AF points have sensors that are arranged either vertically or horizontally, and they are only able to detect details that cross the orientation of the sensor. The horizon in a landscape will trigger a vertical sensor, for example, while horizontal sensors are sensitive to vertical lines – such as the upright edges of a building.
Cross-type AF points are potentially quicker and more precise than standard AF points, because they use both a horizontal and a vertical sensor to detect the subject. Dual cross-type sensors have an ‘x’ arrangement as well as a ‘+’ one, so they are also able to detect diagonal lines of contrast for greater precision.
If your DSLR is struggling to focus in a situation, then selecting a cross-type or dual cross-type AF point can help. The centre AF point in all current EOS DSLRS is a cross-type one, but some cameras have multiple crosstype AF points – the EOS 90D’s 45 points are all cross-type, for example, with the centre point being dual cross-type.
Ultimately, the number of cross-type and dual cross-type AF points that you’ll be able to use when shooting will vary according to the lens that’s being used at the time. You will usually see an f-number listed with the AF point information. So, the EOS 90D’s 45 crosstype AF points are all f/5.6, with the centre point being f/2.8 and f/5.6 dual crosstype. This f-number indicates the maximum (largest) effective aperture that’s required to unlock the additional precision.