Tutorial: Focus

Focusing your EOS

Your EOS camera features an advanced autofocus (AF) system that lets you concentrate on composition whilst it ensures that your photo is sharp. But have you investigated how flexible the system is?

This tutorial will explain how AF works, how the options available will help you take control and create better photos:
• How Auto Focus works
• Using AF points for better focus
• Why to change AF Modes
• AF-assist beam
• Focusing in Live View
• Occasions when Manual Focus may help
• Quick tip: Adjust the Viewfinder to your eyesight

How Auto Focus works

Inside your EOS camera, just behind the lens mount, is a mirror angled at 45 degrees. This reflects light from the lens up to the viewfinder. However not all of the light is reflected upwards. The centre of the mirror is semi-silvered, which means that some of the light passes through.


Angled behind the main mirror (1) is a smaller secondary mirror (2). This reflects light down to an AF sensor (3) in the base of the camera. This sensor looks for repeating patterns and is able to measure the amount of defocus in an image. Data is sent to the focusing motor in the lens to bring the image into focus.

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