How to use autofocus with moving subjects
Taking sharp pictures of a moving target can be a real challenge for photographers of all levels of abilities. In our latest guide of advanced camera tips and expert advice we answer some of the common questions about how to use autofocus with moving subjects and explain how to choose and use the right AF mode.
OK, so remind me, which focus mode should I use for moving subjects?
Your DSLR features two basic autofocus modes: one for shooting stationary subjects and one for moving subjects.
Single-shot autofocus mode, aka One-Shot AF (Canon DSLRs) and Single-servo AF (Nikon DSLRs), is the best mode for subjects that aren’t moving – and in this mode the camera won’t be able to take a picture until the image is in focus. It’s well suited to a wide range of subjects, from portraits to close-ups to landscapes.
The downside is that once the focus is locked at a certain distance, it stays locked there for as long as you keep the shutter release half-pressed. If the subject you’re photographing suddenly moves closer to the camera or further away from it, then they’ll drop out of focus and appear blurred.
The only way around this is to take your finger off the shutter release and half press it again to trigger the autofocus system. As you can imagine, repeatedly doing this to
keep track of a rapidly moving subject soon becomes tedious.
Choose the continuous autofocus setting on your camera, also known as AI Servo AF (Canon) or Continuous-servo AF (Nikon), and the camera will continuously adjust the focus of the lens while light pressure is maintained on the shutter release.
If the subject moves before you take the shot, no problem – the focus system will continue to track it right up until the moment you release the shutter.
Will my camera switch between these two autofocus modes?
No. You’ll have to do it yourself. Many cameras feature a button marked ‘AF’; by pressing this and then rotating the camera’s main dial, you can switch between these autofocus modes.
More advanced cameras, such as the Nikon D300s, feature a dedicated focus mode switch you can use to set the focus mode directly: Continuous (C), Single (S) or Manual (M).
That said, camera manufacturers appreciate that newcomers to photography might not be able to predict when they need to change the autofocus mode, so many models feature a mode that automatically switches between the two.
Set your camera to AI Focus AF (Canon) or Auto Select AF (Nikon) and it will automatically shift from single focus to continuous focus when it senses a moving object.
So, if I spot a moving target and select the continuous focus mode, the camera will automatically focus on it?
It will, as long as the moving target is covered by an active focus point. Cameras have multiple focus points arranged around the centre of the frame, and you can select just one or a small number, or activate them all.
The greater the number of focus points and the more densely packed they are, the more chance there is of the focus system tracking a subject accurately as it moves around.
But there are advantages to manually selecting an individual focus point instead. If all the focus points are active and you’re taking pictures of a subject that isn’t very big within the viewfinder, or the scene has lots of confusing detail in the background or foreground, then the focus system may have trouble locking onto and tracking the subject.
Manually selecting a single focus point means the camera will only focus on the area you choose. This means you have to move the camera in order to keep the focus point locked onto the subject.
Isn’t it hard to keep a moving subject in focus using a single, small AF point?
Yes, it can be tricky if it’s moving fast
– or erratically! This is why cameras come loaded with advanced AF point selection options.
Take the Nikon D5100, for example. It features four AF-area modes: Single Point, Dynamic Area, Auto Area and 3D Tracking.
In Single Point, simply choose the AF point from all of the available ones. Dynamic Area gives you more flexibility. 3D Tracking mode lets you highlight the subject using one AF point, with the camera automatically switching to a different AF point to keep them in focus.
Auto Area makes all autofocus points active and automatically selects what will be in focus.
Should I use continuous drive mode when I use continuous autofocus?
Yes, using your camera’s continuous drive mode enables you to capture a sequence of shots when the shutter release is pressed. The speed at which the camera can initially focus on a moving subject is also key.
The camera starts taking pictures when you fully press the shutter release in continuous focus mode, regardless of whether anything is in focus or not. With a fast-moving subject, there can be a delay as the autofocus tries to lock onto the subject and begin tracking its movement.
There are ways to improve this. Using a lens with a maximum aperture of f/2.8 can make use of a camera’s more sensitive ‘cross-type’ autofocus sensors for improved accuracy.
Pre-focusing the lens at a distance at which you expect the subject to pass also reduces the amount of work the AF system has to do. Some lenses even have a focus limiter switch, which allows you to restrict the distance over which the lens will focus.
But there will still be some instances where it pays to use manual focus.
Switch to manual focus for action photography? Really?
Even ultra-slick focus systems can struggle to keep every frame in focus. In motorsports, for example, cars can hurtle towards the camera at a higher speed than the autofocus can keep up with.
With predictable action like this, it often pays to pre-focus on a spot using single-shot focus, then switch to manual focus to lock the focus distance in.
With the fastest drive mode selected, fire a burst of shots at the optimum moment when the car arrives at the focus distance, maximising your chances of at least one frame being sharp.
PAGE 1: Common questions about using autofocus with moving subjects
PAGE 2: Focus modes explained
PAGE 3: How to set up your camera for action photography
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49 awesome photography tips and time-savers
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on Tuesday, September 4th, 2012 at 11:21 am under Photography Tutorials, Tutorials.
Tags: camera tips, DSLR tips, focus modes, How to focus