At first glance, focusing seems pretty easy, right? You just point your camera, half press the shutter button until you hear a beep, and then take the shot. While at its simplest focusing can be this straightforward, if you want to harness the sharpest shots from your camera it takes a little more consideration and understanding of how the autofocus system works.
So with that in mind, we’ve put together these expert tips to help you fine tune your focusing skills and capture crisper frames.
Mastering your camera's Autofocus system will help you capture sharper shots
Which mode to use
The first thing to do when setting up your camera’s autofocus is to choose either the single or the continuous focusing mode, as they are designed to work best with different types of subject.
The single focusing mode is built for use with stationary subjects, like portraits, landscapes and architecture. The camera focuses when you half press the shutter, and as soon as a sharp focus is set it remains locked for as long as you’re holding the shutter button. If you take a shot or lift your finger off, you’ll need to repeat the process before the next frame. This single focusing mode is referred to as AF-S on a Nikon and One Shot with a Canon. Other manufacturers may share these terms, or use their own moniker, so you may need to consult your manual.
On a Nikon, the Single focusing mode is called AF-S
The continuous focusing mode is designed for moving subjects, such as sports action or wildlife. With this mode, the camera continues to focus for the entire time your finger is half pressed on the shutter button, and is constantly adjusting as the distance between your camera and subject alters. Even as you shoot a sequence of frames, the camera continues to focus. This continuous mode is called AI Servo on Canon cameras and AF-C on Nikon, and once again other brands can vary with their nomenclature.
On a Canon, you need to select AI Servo from the AF menu to activate continuous focusing
How to choose your AF points
Once you’ve chosen whether you want to use the Single or Continuous autofocus mode, you can configure precisely where in the frame you wish to set the focus. A camera’s autofocus system is comprised of a grid of AF points, and the number of points varies depending on how advanced the system is. More basic DSLRs may only offer 9 AF points, whereas the flagship Sony A9 offers a mind melting 693 AF points. Each one of these AF points that you can see in the viewfinder corresponds to a focusing sensor inside the camera.
It's best to take control and decide where in the frame the focus will be set
The mode most fledgling photographers begin with is Auto. This means the camera decides which AF sensor to use, and often focuses on the subject nearest the camera. While you might sometimes strike lucky with your focusing, the chances are the results aren’t as accurate or consistent as you’d expect, and so the Auto mode is best to be avoided.
The primary way to configure the autofocus is to use a single AF point. By default, this is the focusing sensor in the middle of the frame, but you can move it to any of the available AF points by either using the D-pad or holding the AF selector button and scrolling the command wheel, depending on which brand of camera you use. This is a pretty good mode to use, and it lets you take complete control over where in the frame the focus will be set, such as your subject’s nearest eye in a portrait. Sometimes though if your subject is small you can miss your target, and end up focusing on something in the background instead, resulting in a rubbish shot.
If you miss your focus you'll capture a shot with a sharp background and blurred subject
Another option is to select the Group AF mode, which uses a cluster of neighbouring AF points to effectively create a single larger focusing area. This can make it a bit easier to find the focus – a bit like hitting a target with a shotgun rather than a rifle – but lacks the precision of choosing a Single AF point as it tends to set the focus on the closest object within the cluster of AF points. This mode is good if your subject is relatively small in the frame, as it essentially magnifies your focusing area. Again you can shift this group around the frame using the D-pad or Command wheel.
On a Nikon, adjust the AF area using the D-pad
A fourth mode is often available when using Continuous autofocus, and is commonly referred to as 3D/Tracking. This is designed to keep track of moving subjects, even as they move across different parts of the frame and is ideal for sports or wildlife with an unpredictable nature. When using this mode, once the focus is found by the initial AF point it then ‘hops’ to the adjacent focal point as the subject moves within the frame, so you don’t lose focus for as long as the shutter is half pressed.
The 3D/Tracking Continuous AF mode is ideal for moving subjects like sports action
How it works best in different types of camera
With a DSLR you’ll get the fastest and sharpest results by composing through the optical viewfinder, rather than using Live View. This is because when you use the viewfinder, the incoming light is bounced off a mirror both into the pentaprism so you can see the image, but is also beamed onto a set of autofocus sensors. These sensors use Phase detection, which assesses the incoming light by splitting it into two separate beams, and when these signals are identical the focus is sharply set. On the whole, the Phase Detection sensors are more accurate and efficient than their Contrast Detect siblings. This is the method used when you’re using Live View. The camera’s mirror is locked up and the light is directly hitting the sensor where the Contrast Detect AF points are located. It works by adjusting the focal distance until the maximum amount of contrast is detected in adjacent pixels, indicating that the image is sharp with strong, defined edges.
When shooting with a DSLR, the best autofocus performance is through the viewfinder
Traditionally, Compact System Cameras could only offer Contrast Detect AF systems, which made them slower and less accurate than DSLRs. This is because with Contrast Detect the camera doesn’t measure distances, so it doesn’t know which way the lens elements need to be moved. In effect, it tries every which way until maximum contrast is observed. More recently, CSCs have offered Hybrid AF systems which incorporate Phase Detection too, and towards the top end have pretty much caught up DSLRs in terms of autofocus performance.
Putting it into practice
While focusing can be a little more complex than simply pointing, shooting and hoping, it only takes a simple, two-step process to ensure the images you take are strikingly sharp. So next time you’re shooting, first decide if it’d be best to use your camera’s Single or Continuous focusing mode, depending on the chosen subject. With this set, you can configure the AF points to make it easier for you to nail the focus, and you’re good to go.
Four quick focusing tips
1) If your camera’s autofocus isn’t responsive, check it isn’t accidentally switched to manual. First take a look at the switch on the lens and make sure it’s set to the AF as opposed to MF setting; this is most often the cause of a disabled focusing system. If it’s not that, check it’s not set to Manual focus on your camera body’s controls, and that the lens is properly clicked in.
First check the lens isn't accidentally set to Manual focus
2) The high pitched beep the camera emits when the focus is set can be helpful, but may be detrimental for some subjects, such as wildlife or when shooting discreetly. It’s often best to disable the beep in the camera menu, you’ll soon get used to shooting without it, and it’s less annoying for other people.
3) Focusing can sometimes struggle, it rocks back and forth trying to find the sharp spot, and this is known as ‘hunting’. This mostly happens when there isn’t enough contrast, either because a subject lacks any definable edges or there isn’t enough light. If this happens, try focusing elsewhere on an area of higher contrast.
4) If your camera is really struggling to set an accurate focus – maybe because it’s a very low light scene – it’s best to take complete control and set it manually. To do this, set the switch on the lens to the MF position and turn on your camera’s Live View. Zoom into the area you want to be sharpest using the magnify button, and rotate the focus ring on the lens to adjust the sharpness.
. digitalrev.com2018-2-22 03:00
