Focus modes explained
When you look through your SLR’s viewfinder and half-press the shutter button, you’ll see the multiple autofocus points flash when they’ve achieved focus, and you may also hear a beep – depending on the make and model of your camera. These cues are designed to help you take sharp photos. However, if you shoot using basic shooting modes (such as portrait, landscape, sports, and so on), most SLRs will automatically select the AF point, and will then use this AF point to focus on what it thinks you want to focus on – usually the thing closest to you, which might not be what you want to be in focus at all!
Points of interest
However, one of the many advantages of using an SLR is that you can control what your camera focuses on by selecting your AF point manually. This means – when shooting portraits, for example – that you can focus on one of your subject’s eyes, even if it’s off-centre in the frame. The number of focusing points you have varies from camera to camera. The Canon EOS 1100D, for example, has nine AF points, the Nikon D5100 has 11, the Canon EOS 7D has 19, and the Nikon D7000 has 39.
Control the focus
To manually select individual AF points, press the AF Point Selection button on your camera and then look through the viewfinder. Use the top dial, crosshair buttons or joystick to cycle through each AF point until the one over your chosen subject is highlighted. Half-press the shutter button to lock the focus, then fully press it to take the picture.
Helpfully, the AF points are placed on (invisible) vertical and horizontal lines at points one-third into the frame, so they can also help with composition when it comes to the rule of thirds – use them as guides for placing the subject in the frame. Note that if all the autofocus points light up at the same time, you’re on Auto Point Selection.
Single Point AF
This mode is best used when shooting a portrait, say, or any subject that stays relatively still. It enables you to select a focus point manually. By default, the centre focus spot (which is the most accurate) is used for focusing in single point autofocus.
Dynamic Area AF
This mode is perfect for shooting fast or erratically moving subjects. Some SLRs have up to 50 autofocus points – you choose one, but if the subject moves away from that point, the camera will continue to focus using one of the adjacent AF points.
This is the most commonly used autofocus mode. All you need to do is point the camera at the target, half press the shutter-release button and the lens will focus. Crucially, the focus point will remain locked for as long as you maintain the half press on the shutter release button, enabling you to recompose the shot while keeping the selected point in sharp focus. You can only shoot once the focus is locked, but you can usually override this with a setting in the camera menus, should you wish to.
This is more suited to moving targets, such as sports or wildlife subjects. The main difference between this and one-shot autofocus is that the lens continuously focuses on your subject. The autofocus system tracks the subject to ensure that it’s sharp in the captured image.
By default, continuous mode enables you to take a shot whether focus is achieved or not. There are limitations, such as in motor sport, where a car comes towards you at a higher speed than the autofocus can keep up with. In situations like this, it’s best to pre-focus in one-shot mode.
In this ‘intelligent’ autofocus mode, often referred to as ‘auto-select’ or AI Focus, the camera’s autofocus system automatically senses whether an object is stationary or moving, and then switches between one-shot and continuous modes.
Low-light conditions, subjects with fine detail, and shooting through glass are all situations where even the most costly lenses will struggle to focus. If autofocus fails to lock on to an edge, simply switch to manual focus and turn the focus ring until your subject is sharp.
The camera exposes a single frame each time you press the shutter release. This is best used for still subjects such as landscapes and portraits, where you don’t need a burst of frames to catch a fleeting moment.
In this mode, your camera will fire off shots
at its maximum frame rate for as long as the shutter button is held down. It’s ideal for sports, but you need a memory card with a fast write speed for long bursts.
With typical options of two- or ten-second delays, the self-timer mode can be used to fire the shutter remotely when, say, your SLR is mounted on a tripod. This enables you to press the shutter before positioning yourself in the shot, or to reduce camera shake.
Back to: Perfect your exposures
Forward to: Choosing a lens
on Thursday, December 8th, 2011 at 11:05 am under Photography for Beginners.
Tags: Auto-Select/AI Focus, Continuous/AI Servo, drive modes, Dynamic Area AF, Manual, photography tips, Single Point AF, Single-Servo/One shot