How Does Auto-Focusing Work?

Photography Tips

The science behind camera auto-focusing systems can be an intimidating subject for a beginner. In this short video Techquickie attempts to unravel the mumbo-jumbo:

There are many different types of auto-focusing systems. They can be broadly broken down into two basic types: active and passive.

Spoiler: All DSLR cameras use one or more forms of passive auto-focusing, and so do other digital cameras

Passive Autofocus

Passive Autofocus Systems

Passive systems are named as such because they use a technology that fundamentally adjusts for focus after an image has been projected onto one or more sensors. The camera then adjusts focus based on a series of algorithms that help it to determine the distance between the subject and itself. Passive systems are available in two major varieties: phase detection and contrast detection.

1. Phase Detection

Phase detection systems, which are found in DSLR cameras, use a beam splitter to project the oncoming light onto two different sets of sensors.

Beam Splitter

This creates two images which are then matched using the camera’s software. It matches the light intensities in the image to establish perfect focus. In older days, this would have been done manually (rangefinder cameras).

2. Contrast Detection

Contrast detection, on the other hand, measures the pixel intensities of the image on the sensor.

Contrast Detection

The auto-focusing system then adjusts focus, first by pushing the image beyond the point of maximum contrast and then bringing it back to proper contrast level. This results in brief focus hunting. You’ve probably noticed this when shooting videos or stills in live view mode on your DSLR or shooting with point & shoot cameras using the LCD monitor—the subject gets in and out of focus briefly.

Active Autofocus Systems

Active auto-focusing systems are more complicated, as they tend to actually measure the distance between the subject and the camera. Active systems function beyond the scope of the normal optical systems.

So far only two applications of active systems have been used. One of them uses sonar and the other uses infrared beams of light. The impracticality of using these systems in camera systems notwithstanding, there are other drawbacks. For instance, active systems fail when you shoot through glass or underwater.

Hopefully this gives you a better understanding of how your camera’s focusing system works!