Focusing errors can ruin even the best composed images. In this quick guide we answer some of the common questions people have about how to use autofocus in their everyday photography.
Which focusing option should I choose for everyday photography?
The latest DSLRs feature a wide array of options for autofocusing, but ultimately focusing boils down to three choices: single shot, continuous or manual. The terms used to describe these modes vary between camera makes, but they all do the same thing.
Single shot autofocus (AF), also known as Single-servo AF (Nikon) and One-Shot AF (Canon), is designed for stationary subjects.
It’s the default AF mode – the everyday choice – in which the camera achieves sharp focus when you half-press the shutter-release button, and locks it for as long as you keep the shutter release half-pressed.
This enables you to recompose the image while keeping the initial point in sharp focus (Single shot autofocus shouldn’t be confused with Single Shooting drive mode, which is Canon’s term for shooting one image at a time.).
Continuous autofocus, also known as Continuous-servo (Nikon) and AI Servo (Canon), is the best mode to use for moving subjects.
Rather than locking the focus on a fixed point, the camera continuously adjusts the focus to track moving subjects. We’ll be looking at moving subjects in more depth next issue, but again, continuous AF shouldn’t be confused with Continuous drive mode, which is Canon’s term for its burst option.
Manual focus, as the name suggests, means that all focusing is done by hand (and eye). It’s useful in low light, when the camera’s AF system can often struggle to lock onto a subject, or when subtle focus adjustments are required, such as in macro photography.
You’re still not on your own though: an LED in the viewfinder, and sometimes a beep, will let you know when the camera calculates that you’ve achieved focus.
OK, single focus for stationary subjects, continuous focus for moving subjects. But my camera has another autofocus mode – what’s that for?
Many cameras have a third autofocus option, called Auto Select on Nikon cameras and AI Focus on Canon cameras. This isn’t a dedicated focusing system per se, but a focus mode that automatically switches from single shot focus to continuous focus when it detects a moving subject.
If you’re unsure which option to use, it’s a good starting point. However, as with all auto modes, you’re handing the decision-making over to the camera – and it might not select the option you want.
Is that why some of my pictures don’t look sharp?
There are several factors that play a part in determining how sharp a picture appears, such as the quality of the lens, depth of field, and how well supported the camera is.
However, accurate focusing is critical, and your camera has a range of tools to ensure this happens. A dedicated autofocus sensor inside the camera analyses the contrast in the image, and adjusts elements within the lens until the shot is in focus (see box, top left). However, there are some situations where it can get it wrong.
Why does single shot autofocus come unstuck?
Your camera has no idea which part of the frame you want to focus on. In fact, in its full auto mode, it will usually try to lock onto the part of the picture that is closest to the camera.
Even professional DSLRs featuring more than 50 AF points spread over a wide area can and do lock onto the wrong subject when left in the default automatic AF point selection mode.
You can get around this by manually selecting the central focus point as your default or by selecting a focus point that lines up with the part of the scene you want to be sharp.
I did that, but the lens just kept focusing backwards and forwards. What’s happening?
Autofocus can fail when faced with low-contrast subjects, such as a white wall or a sandy beach. AF sensors use contrast as the basis for determining sharpness, and they adjust the lens elements until they lock onto a high-contrast detail.
Low-contrast subjects provide little to lock onto, and as a result lenses can tend to ‘hunt’ for a focus point.
Taking your finger off the shutter release button and half-pressing it again can sometimes reset the system and rectify the problem, but it’s often easier to focus on a higher-contrast part of a scene that’s the same distance away as the subject.
My camera seems to struggle to focus indoors too. Is that because of the same problem?
Yes, lower light levels lead to lower contrast. That’s why cameras have AF assist lamps, or fire their pop-up flash, in order to give autofocus a fighting chance in the dark. Lenses with wide maximum apertures are handy here, as they allow more light to reach the autofocus sensor.
It doesn’t matter what aperture you have set, as the camera will only set this aperture when the shutter release is fully pressed. The image you see through the viewfinder is always at the lens’s maximum aperture.
So why do pictures taken with my f/5.6 wide-angle look sharper than those with my f/4 telephoto?
That is probably due to the greater depth of field offered by the wide-angle lens. Depth of field is dependent on the aperture used, the distance the lens is focused at, and the focal length of the lens.
In more practical terms, shooting a distant subject using a wide-angle lens at a very small aperture will produce shots with everything sharp from front to back.
Conversely, using a longer focal length to shoot an object close to the lens at a very wide aperture will result in an image in which only an extremely narrow band is in focus; in this case, the focusing has to be very precise to ensure the right part of the subject is sharp.
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