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Apr 21, 2017 // 12:37 PM

Canon DSLR Focusing Basics - How to choose the right AF mode

Written by Mike Duval

For photographers, there’s no part of a camera that’s more of a mystery than the autofocus system. Not many people fully understand how it works. If you’ve ever found yourself let down by your camera’s autofocus, there may be a good reason why. In the first post of this multi-part series, we’re going to look at the autofocus system of Canon cameras, specifically the awesome Canon 5D Mark IV, and break down the various modes and focus point options selection options so you can get better results.

Canon 1DX Mark II AF Sensor.jpgThe Canon 1DX Mark II and 5D Mark IV share the same AF sensor with 61 AF points.

How does autofocus work?

For still photography, cameras like the Canon 5D Mark IV and other DSLRs rely on what’s called a phase detection autofocus system. Without getting too technical, the basic idea is that the camera uses a unique mirror to split light for autofocus sensors to read. For this to work properly, there has to be enough detail & contrast for the camera’s AF sensors to focus properly.

Ever have trouble with your camera trying to focus on a blank wall or something with little to no detail? Did your camera’s autofocus hunt back and forth or seemed confused? This hesitation is because the AF system doesn't have enough detail to make a focus adjustment.

The trick is to make sure something in your frame has distinct lines or contrast and line up your focus point on that area. Good examples would be the edge of a window or the corner of an eye. This will give your camera the detail and subject contrast it needs to achieve focus. Just knowing that this is how a DSLR camera's autofocus system works is a huge step to getting more accurately focused images.

The Basics: AF Operation Modes

Once you know that your camera is looking for areas of contrast and detail to focus on, the next step is deciding how you want the camera’s AF system to act. The most crucial setting for this is the autofocus mode. There are three different modes and you may already be familiar with them from the top LCD of your camera. 

Canon 5D Mark IV Focus Modes Top LCD Gif.gifOne-Shot - Focus, lock, and stop

AI Focus - See below.

AI Servo - Always trying to focus

First off, we can totally forget about AI Focus AF. It's the village idiot of focusing modes. In AI Focus AF, the camera is left to make all the decisions about subject movement and when to stop or start focus. The computer inside your camera isn't smart enough to make these decisions, so we'll pretend this mode doesn't exist. Trust me, your photos will be better for it!

That leaves us with two AF modes to remember, One-shot and AI Servo mode. Both will focus the camera when you half-press the shutter button; the difference is what happens after the achieves focus.

ONE Shot AF Mode

Canon One Shot AF Mode - GIF.gifIn One-Shot mode, the camera stops focusing once it achieves focus until you release the shutter button. This is a great mode for people who like to focus and then recompose their shot. You also get a visual (or auditory) confirmation that the camera has achieved focus. The other behavior by default is that your camera won’t take a photo unless the camera thinks its achieved focused on something. As long as you and your subject are not moving, it works great. If either of you is, then you want AI Servo.

 

AI SERVO AF Mode

Canon AI Servo AF Mode - GIF.gifAI Servo AF is designed to continue to focusing as long as you’re half-pressing the shutter button. This means that if you or your subject is moving, the camera will continually try to keep it in focus. Unlike One-Shot mode, AI Servo mode prioritizes the release of the shutter, so you never miss a moment, but it will it take a photo whether your subject is in focus or not.

There are a lot more things that go into a complex auto focus system like in the Canon 5D Mark IV but only knowing that your camera needs detail or contrast to focus and the two focus modes will give you so much more control. Stay tuned for future blog posts on how to choose the right kind of focus points and how to adjust the AF custom settings for even better results for everything you shoot.

Give these two modes a try and practice focusing on various objects in different shooting situations. In our next post we'll tackle the 61 Focus point system of 5D Mark IV and the various styles and layouts you can choose depending on your subjects. 

Topics: Photography


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