Camera College
Digital cameras have multiple focus points, and often multiple focus point arrangements to choose from. But how exactly do they work?
Everything you’ll ever need to know about autofocus, over six pages
Your camera’s autofocus or AF system is a complex and sensitive beast, but it can be broken down into three simple steps: how, when and where. The AF mode you’ve set on the camera determines ‘how’ the camera focuses – whether it stays locked on, continuously adjusts the lens when it detects movement, or does a bit of both.
‘When’ is often the point at which you half-press the shutter button or tap a camera’s touchscreen to take a picture, but it doesn’t have to be: you can customise a camera’s control layout so that autofocus and the shutter release functions are assigned to different buttons and carried out independently. This feature is useful when you quickly need to pause continuous autofocus operation while you continue to take pictures.
But the crucial step in the whole AF process is ‘where’. If the camera isn’t focusing in the right place, that could mean a shot where the most important details may be slightly fuzzy or even completely blurred. It’s the camera’s AF points that determine which part of the image the camera will focus on, and how precise it is when it does so.
Digital SLRs have two separate autofocusing systems: one for the
Before you take a photo, you need to choose how the camera will record the image. We’ve explored the two most common file formats – Raw and JPEG – in detail in recent issues, but the key difference between them is that a Raw file is not an image, and has to be processed to create
optical viewfinder, and another for Live View shooting with the rear display. When you shoot with the optical viewfinder, the camera uses a dedicated AF module to work out when the image is in focus. The AF points you see in the viewfinder correspond with the position of the focus detectors on the AF module; you can manually select a single AF point, or select them all and let the camera decide which AF points to use within the array.
More advanced cameras also enable you to select small groups or zones of AF points. If you activate all of the AF points (or a group of them, in some cases), then the camera will typically focus on the nearest part of a scene or the area of highest contrast.
As you might expect, the greater the number of AF points, and the wider they’re spread across the picture area, the easier it is for the camera to focus on details and objects that aren’t in the middle of the frame. A densely packed array of AF points will also make it easier for the camera to keep subjects that are moving in focus, as it can pass the autofocusing duties from one AF point to a neighbouring one.
When a digital SLR is used in Live View mode, the mirror assembly is moved out of the light path so that the main sensor can be exposed and an image can be displayed on the rear screen. Without the mirror in position, no light is reflected down to the camera’s dedicated AF module; as a result, it can’t provide the ‘eyes’
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of the autofocus system. Instead, the camera has to use the image on the main sensor to work out what is sharp. This can be slower than using a dedicated AF sensor, as the camera has to focus the lens back and forth to detect when contrast is at its highest (giving the sharpest result).
That being said, the Live View display offers many more AF points than an optical viewfinder, covering the image almost from edge to edge. Some cameras also have pixels on their imaging sensors, which act like traditional AF points and speed the whole process up.
Mirrorless cameras are essentially always in Live View mode, with the live feed from the imaging sensor being displayed on the rear screen or in an electronic viewfinder (EVF). As a result, an EVF is more versatile than a traditional optical viewfinder, able to offer features such as magnifying the point of focus to check that it’s sharp.
The AF system of a mirrorless camera also tends to be more sensitive, meaning it can autofocus in darker conditions. Autofocus sensitivity is expressed as an Exposure Value (EV) range, or ‘stops’, with the first number indicating how low it can go. Compare the Canon EOS 90D (DSLR) and the EOS M6 Mark II (mirrorless): these 32.5MP camera cousins share a similar feature set, but while the 90D focuses from -3 to 18 EV, the M6 Mark II focuses from -5 to 18 EV. There still needs to be some contrast for the system to work.
Although off-centre AF points make it easier to focus on subjects that aren’t in the middle of the picture, this won’t always be the case. Sometimes the subject may not line up with an AF point. In this case, move the camera so it is lined up, focus on the subject, then keep the shutter release half-pressed to lock the focus while you recompose your shot. If you’re shooting at close distance with a wide-angle lens, there’s a risk that the subject may not be as sharp as it would be if you didn’t have to move the camera.
If the camera struggles to focus with an outer point, try using the more sensitive centre point.