Canon Auto Focus Modes and Back Button Focusing Explained

November 22, 2011 · 3 comments

in Animals,Equipment,Product Reviews,Tutorials

There are three types of Auto Focus modes on a Canon dSLR camera:

One Shot, AI Focus and AI Servo. To set the required Auto Focus mode, first ensure the focus switch on the LENS is set to AF. Next press the AF Drive button on top of the camera. Turn the dial on top of the camera to select the desired mode.

Which focusing mode is best for me?

One Shot AF is best for still subjects. When you press the shutter button half way, the camera will focus only once. When focus is achieved, the AF point that achieved focus will flash red, beep, and the confirmation light will light up.

With evaluative metering, the exposure setting will be set the same time focus in achieved.

When you hold down the shutter button half way focus will be locked, meaning you can recompose the shot if required.

AI Servo AF is best for moving subjects and when the focusing distance keeps changing. When you hold the shutter button down half way, the camera will continuously focus on the moving subject. Exposure is set at the moment the picture is taken.

When the AF point selection is automatic, the camera first uses the centre AF point to focus. Within the spot metering circle, there are six invisible Assist AF points that function in the AI Servo mode. So even if the subject moves away from the centre AF point, focusing can still continue. If the subject moves out of this range, focus tracking will continue by using another AF point.

Hold the shutter halfway down and get a focus lock on your subject, then follow your subject while still holding the shutter halfway down until the shot is what you want, and then fire away! The camera will constantly adjust focus while you follow your subject.

With AI Servo AF, the beeper will not sound when focus is achieved, and the focus confirmation light will not light. This can be a little deceiving, so reply on your eyes to check if you achieve focus before taking the photo.

Image captured using AI Servo AF and Back Button Focusing, photographed in MalaMala

AI Focus AF automatically switches from One Shot AF to AI Servo AF if the subject starts moving.

When focus is achieved in the AI Servo AF mode (with the Servo mode active) the beeper will beep softly when focus is achieved, however the confirmation light will not light up.

Back Button Focusing: The AF-ON Button

In P / Tv / Av / M and B settings, Auto Focus can be achieved by pressing the AF-ON button, instead of pressing the shutter button half way. This is especially handy when photographing moving subjects with AI Servo, but can be used with any of the Auto Focus Modes. Essentially, the AF-ON button allows you to set focus before depressing the shutter button.

Back Button focusing can be a little tricky to get used to, but when you get the hang of it, you’ll never look back!





{ 2 comments… read them below or add one }

Tom K. June 30, 2012 at 6:00 am

Excellent explanation. By the way you have a beautiful portfolio.

rikkihibbert July 2, 2012 at 8:27 am

Thanks Tom!

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