Contrast and phase detection in cameras

- Different cameras have different autofocus features. Some autofocus systems can detect faces, or track moving subjects. Some work better in low light than others. Most cameras these days have several different autofocus modes and options available, and there's a lot of variation in the interfaces and feature sets of the autofocus systems that you'll find on different cameras. On most cameras today, autofocus systems are built on top of two different passive autofocus technologies. Passive means they analyze light coming into the camera to determine focus.

This differs from active focus technologies that emit something like sound or infrared light to measure focus. Early autofocus systems were generally active, but today's passive systems yield better focus with fewer drawbacks. Active systems can be confused by when you're shooting through glass or bars on a cage at the zoo, they usually don't work well under water. If you're using a moderately recent camera, then it probably uses one of two passive systems, either contrast detection autofocus, or phase detection autofocus.

Now, there are also some cameras that will use a combination of both. We'll talk about that later. To get good results from your autofocus system, you don't actually have to know what kind of technology your camera uses. However, a little understanding of the differences in these technologies will help you when you're shopping for a camera, and might help you get better results when you're shooting. Contrast detection is the more prevalent system. It works by analyzing in real time, image data that the camera is capturing, and it exploits the fact that an image with soft focus has less contrast in it, than an image that is sharply focused.

So, the camera focuses to a particular point, samples a bit of image, analyzes it, then focuses to a different point, analyzes that, and if contrast has increased, then the camera knows it is changing focus in the right direction. So it continues in that direction, doing that same thing, until contrast decreases, and then it pulls back. And in that way, it can slowly zero in on the correct focus distance. If you've ever heard your lens focusing back and forth as it hunts for correct focus, then you have a contrast detection autofocus system, and you've experienced one of the potential drawbacks of such a system.

They can be slow as they hunt for focus, and they can be noisy. What's more, if you're shooting a low contrast scene, they may simply never find correct focus. If your camera is hunting for focus, consider what you're pointing at. If it's a bright, light, sunlit wall, there's not much contrast there. Try to find an edge or a seam at the same distance that you want to focus on, and put the camera's focus point on that. The camera will probably then be able to find focus. On the upside, contrast detection systems are inexpensive to implement.

The actual technology doesn't take up much space or weight inside the camera. And when they do have a regularly contrasty scene to work with, they can be extremely accurate. Phase detection also works by analyzing light coming in to the camera, but instead of analyzing contrast, these systems analyze phase shift. The easiest way to understand this at a very simple level, is simply to hold your finger up in front of your face. If you focus your eyes on something far away like I'm doing now, your finger splits in two.

If you rejoin those fingers, and you'll see my eyes move closer together as I do that, if you rejoin those fingers, if you bring them back into phase, your finger comes into focus. This is what phase detection autofocus does. And it requires a beam splitter to get light split into two different places. And it needs sensors at those locations. All of this takes more room and more components. And so, it's bulkier and more expensive. However, it's extremely fast. And it doesn't require that hunting process the contrast detection uses.

In some systems though, phase detection is not as good in low light as contrast detection. In other systems, it's superior to contrast detection in low light. One interesting note about your eyes. When you focus your eyes to rejoin your finger in front of your face, everything in the background splits in two, but your brain just ignores that for you so that you're not distracted by it. If you really pay attention, though, as you do this, you'll realize that the background is doubled as you split your finger apart. As you move through the world, you're actually always either seeing double in the foreground if you're focused far away on something, or double in the background if you're focused on something close up.

The fact that you don't notice this is a testament to how much your brain is mucking with your vision, so that you don't notice the constant double vision that you're actually seeing. Anyway, if you're shopping for a camera, what system should you look for, contrast detection or phase detection? There was a time where you could say, oh, phase detection is definitely the more advanced, superior technology. And in that time you'd say it in some weird voice for some reason, but anyway, advances are always being made in both contrast detection and phase detection. So don't be swayed simply because the camera touts a particular technology.

Rather, look for real world reviews of a camera's autofocus and base your decision on the conclusions of actual autofocus use. Now as I mentioned before, some cameras might employ both systems. For example, this Fuji XT1 has both contrast detection and phase detection, but it has more contrast detection. Like most cameras, the XT1 has a number of autofocus points that you see in the viewfinder, and you can choose the one you want. Some of those points employ contrast detection technology, while others employ phase detection.

The points in the viewfinder actually look different depending on what technology they use. Phase detection is going to be better for motion tracking, so when using that feature, I want to be sure to place the phase detection points on my subject. Contrast detection might be better in low light, so if I'm having trouble focusing with the phase detection points, then I might want to switch the camera to one of its contrast detection points. If your camera offers both, then you're gonna want to figure out what the difference is, what they are, what they look like, and learn to use them accordingly. So, anyway, again, a deep understanding of these technologies is not critical to knowing how to use your camera.

But when it comes time to buy, some of the specs you're seeing might make a little more sense now, and in certain situations, knowing whether you have contrast detection or phase detection, may help you figure your way out of a situation when the camera can't focus.