In situations where you are relying on your camera’s autofocus, it’s very important to know how the focusing points work.
There are major differences between camera models and you should always find out about yours before you start using it. The focusing points (whether your camera has 9 or 19) are not all the same. Your cameras ability to focus accurately depends on whether you are using a single or a cross-type focusing point when focusing and composing your image. This article over at Digital Photography School explains the issue in detail.
The problem with traditional phase-detecting systems is they get a bit stumped if there are a lot of vertical lines in the spot where they are trying to focus. To see how this works for yourself, print a sheet of paper on your computer, with nothing but vertical lines. Tape it to a wall, and try to focus on it with your camera. If you are using one of the focusing points on the outside edge of your camera’s viewfinder, your lens will likely spend a few seconds hunting for focus but will probably never find it. However if you turn the paper sideways and try again your camera will likely get things focused fairly easily. This is because when light is sent to the phase-detection sensors in your camera, the sensors don’t have enough information to determine focus, if all it sees is vertical lines.
While most of the time when you are out taking pictures, you are probably not shooting images of vertically-lined paper, this example does illustrate how your camera’s autofocus can get slowed down, and become unreliable under certain conditions. Ironically, in this test, your camera will find focus much better if you use the live view function. That employs a contrast-detection focusing method which is also used in most mirrorless cameras, and while it is a bit slower, can have some advantages over traditional phase-detect systems.
To address this issue, most camera manufacturers have implemented cross-type focusing sensors that work fine when focusing on images with horizontal and vertical patterns. On high-end models (like the Nikon D5 or Canon 5D Mark III) there are several clusters of cross-type focusing sensors, but lower-end models (like the Nikon D3200 and Canon Rebel T3i) usually have just one, right in the center. This means that the center autofocus point will likely be significantly faster, and more reliable, than the points on the edge. You can see the results yourself by repeating the test from earlier with the center focus point, instead of one on the perimeter of your viewfinder.
The real-world implications of this are quite significant, and may very well change how you approach your photography. Many people use an automatic setting that allows their camera to look at all the available focus points, and determine which one should be used to set the focus. But, if you know that the the cross-type points will give you consistently better results, you might try using them more often.
Of course all this doesn’t mean that the normal focusing sensors on your camera are worthless, just that knowing which ones are cross-type can often give you an advantage you might not have otherwise had.
If you’re having a hard time understanding what all this actually means, we have a clever video for you that explains it very clearly in less than 2 minutes.
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Read the full article over at Digital Photography School.
Source: Digital Photography School
