DSLR Autofocus – How and Why it Works (or Doesn’t)

There are few things more frustrating to a photographer than when their camera won’t autofocus or when it gets the focus wrong. In this article we’ll take a look at how exactly autofocus works in today’s digital cameras and what you can do to increase the odds of getting your photos in focus the way you imagined when you made the photograph.

Cameras don’t really understand what is and what isn’t in focus. When you point the lens of your camera at your cat, the camera doesn’t know that it is a cat let alone know that you probably want the cat’s eyes in focus. Instead, modern SLR cameras us a technology known as phase detection. Phase detection works by splitting the light into pairs and comparing these pairs. A beam splitter directs the light to a pair of autofocus sensors and the way the light falls on the sensors determines whether or not the camera has found focus. If not, the camera knows how much out of focus it is. If necessary, the camera makes an adjustment to the focus and checks again.

© Paul Burwell Photography | Burwell School of PhotographyModern cameras use a combination of two-dimensional cross-type sensors that are sensitive to both vertical and horizontal lines and one-dimensional line detectors that are looking for focus on either horizontal or vertical lines. The cross-type sensors are more accurate and faster than their line sensor brethren. The center focus point is usually a cross-type sensor and should be your “go to” focus point when getting accurate focus is critical.

What are the implications of this? Focus is a lot easier for the camera to detect under bright lighting conditions. That’s why many modern cameras utilize either the camera’s flash or a built in LED or infrared light to temporarily illuminate a scene to improve the lighting conditions.

Another implication is that some objects are much easier for the camera to focus on than others. Focusing on the point of the maple leaf in a Canadian flag on a nice summer day is trivial because the camera can easily detect the difference between the red and white areas. Alternately, trying to focus on the dark eye of a black Labrador Retriever in the shade is where the camera can run into difficulty. Without some sort of line to focus on, the camera will struggle. Therefore it might be better to pick a spot just to the left or right of the dog’s eye where there’s a line between the side of the dog’s head and the background.

© Paul Burwell Photography | Burwell School of PhotographyDifficulties can also occur when the photographer takes the area focused on off of the subject. This unintended error often happens when making images of objects in motion. For example, a photographer attempts to maintain focus on a bird in flight while it flies in front of a bunch of bright tree trunks. Unless the photographer is perfect with their ability to match the path of the bird, the area the camera is looking at to focus on may temporarily drift onto those bright trees and guess what? The camera finds some lines that are easier to focus on and adjusts autofocus to that spot. The result is another blurry subject and the sound of a photographer cursing loudly in the woods while attempting to reacquire focus on the bird again.

Here is something to think about the next time you’re deciding between a lens and a camera body purchase: faster lenses (larger maximum aperture) make it easier for the camera to focus. Cameras focus with the lens wide open. It isn’t until the shutter is activated that the lens stops down to the f-stop the exposure is being made at. And while there may not seem to be a big difference between a F2.8 and a F5.6 lens (after all 5.6 is just two times 2.8), in reality a F2.8 lets in a whopping four times amount of light as an F5.6 lens. Four times! So, if you want faster and more accurate autofocus a fast lens will go a long way to helping you out.

Autofocus cameras have been around since the 1970’s and over time the technology has dramatically improved. However, it is still far from perfect and your pictures will benefit from using the information above to help your camera achieve the sort of focus you’re looking for.

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