Here’s How Your DSLR’s Phase Detection AF System Works

Did you know that your DSLR’s autofocus system uses phase detection and that it works somewhat similarly to a rangefinder.

Whether you’re buying your first DSLR camera or finally upgrading to a better model, one of the first things you might be keen on looking at is the autofocus feature. In this very quick but very informative video, we learn how exactly the autofocus works in DSLR cameras, and how this knowledge can help you decide on your next buy.

Autofocus, without a doubt, is one of the coolest and most useful things ever to have come to the world of photography. But how exactly does it work? The short video below by ZY Productions covers everything we need to know about a DSLR’s AF system, and why it’s important to get an understanding of the technology behind it.

Now, wasn’t that very clearly and concisely explained?

As mentioned in the video, the AF module is actually not within the sensor itself or the viewfinder, but somewhere below the mirror. Some of the light that passes through the lens is also reflected onto that module. The technology behind the DSLR’s AF module is called phase detection, and it works quite similarly to that of a rangefinder. When the subject or scene is out of focus, the AF module detects a split image; the farther apart the split image is, the more out of focus the subject or scene.

We also learn that a camera with a vertical AF point isn’t the best for situations where the subject is mostly vertically oriented (like a vertical line in the video’s example) or dominated by vertical lines. The AF struggles to work because it doesn’t detect much difference between a vertically split image and a correctly focused one. This is why the other AF point types were developed: the cross type, which combines two vertical AF points stacked together into a cross, and the dual cross type, which combines two cross type points into an 8-pointed star shape. The former allows the AF system to do its thing vertically and horizontally, and the latter even diagonally as well.

What makes this information relevant to you as someone who either plans to get your first DSLR or upgrade into a new one? Because, again, as stressed in the video, it’s more important to know the type of AF points a DSLR has than the number of points it boasts (which is one of the things mistakenly looked for first). So basically, aim for the cross type AF point at the very least for faster and more accurate autofocus.

Do check out the ZY Productions on YouTube for more camera reviews, tech stuff, and tutorials.


Screenshot image from the video by ZY Productions