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How Autofocus Works

What to know about today’s autofocus technologies

This Article Features Photo Zoom

Olympus E-620 SLRs typically employ phase-detection AF—it’s faster than the contrast-detection AF systems used in all compact cameras such as the Panasonic Lumix DMC-FS15. However, SLRs with Live View, like the new Olympus E-620, will switch from phase detection to contrast detection when in Live View mode.

There are a lot of different technologies that go into the design of digital cameras, and one of the most powerful is a camera’s autofocus system. There are two kinds of systems in use today: contrast-based and phase detection, both of which can be found in digital cameras that range from the basic point-and-shoot to a professional digital SLR. Both are complex systems and vary in how they achieve focus. Understanding how they differ can help you make more informed camera purchases and possibly get better results from the cameras you own.

DIFFERENT PHASES
Phase-detection AF systems are found in SLR cameras. In digital SLRs, phase-detection technology uses an additional sensor (separate from the imaging sensor) to achieve focus. In modern D-SLRs, the reflex mirror is partially transparent so that light coming in through the lens can reflect down onto a sub-mirror, which reflects that bounced light onto the AF sensor.

On top of the AF sensor is a set of precision optics that splits incoming light into two separate beams. Based on where these two beams strike it, the sensor can determine how far out of focus your image is and whether focus is in front of or behind the focal plane. Once the AF sensor does this calculation, accurate focus is established. The calculation is performed in milliseconds and can be done repeatedly at a very fast rate, giving the camera the ability to continually change focus or automatically track a fast-moving subject.

Panasonic Lumix DMC-FS15

CHECKING THE CONTRAST
Contrast-detection autofocus is the primary AF system used in compact point-and-shoot cameras; many D-SLRs also use it for autofocusing in Live View mode. This form of autofocus differs because it uses the same sensor—the image sensor—for both autofocus and image capture. The AF system analyzes the scene and tries to find contrast.

It typically starts with the lens at the infinity position and moves step by step to the close end of the focusing range, gauging the contrast to see if it increases or decreases.

As the contrast increases, the system knows it’s getting closer to an accurate focusing point. Once it passes this point and the contrast begins to decrease, the system has identified the peak of contrast and locks down focus at the peak.

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