In today’s world of responsive technology, shooting successful imagery has never been easier. It’s hard to believe that not so long ago, a jostling Fleet Street photographer would have had no choice but to focus camera manually in the photo scrum, as would sports photographers.
Can you imagine tracking a fast-moving tennis pro without lightning-quick AF? Thankfully, today every camera, from the humble mobile phone to a consumer SLR, has a sophisticated and accurate AF system.
Phase Detection vs Contrast Detection AF
Autofocus (AF) is split into two categories: Phase Detection and Contrast Detection.
Phase Detection AF is used in digital SLR cameras: it takes some of the light hitting the mirror inside and redirects it onto a separate AF sensor. The sensor can calculate whether the subject is in focus very quickly, which makes it highly useful for tracking moving subjects.
Contrast Detection, found in point-and-shoot cameras, uses the actual imaging sensor to calculate focusing. It starts by focusing at infinity, checking the subject for contrast. If the subject is close, it appears very blurry and subsequently lacks contrast.
The camera moves the focusing and as it does so the image contrast increases. This is why point-and-shoot cameras operate far more slowly and often less intelligently than DSLRs.
What are camera focus points?
Inside a modern DSLR is an array of focusing points that can be seen through the viewfinder window. These autofocus points are sophisticated aids that allow the focal point of the image to be positioned by the photographer according to the subject.
A central focusing point is of little help to a portrait photographer, who relies on a left or right bias to position their subject’s face accordingly. When shooting moving kids in particular, in-camera technology is extremely helpful.
Once focus lock is attained using the centre spot, if the subject moves around the frame, the position is passed on to the other points.
Static and Continuous focus
Static or continuous AF modes give you a choice of subject-dependent tracking options. One-shot modes, for example, are great for static subjects like portraits, while Servo focusing focuses continuously with the shutter half depressed.
The AI Focus mode offers the best of both worlds, switching between the two modes automatically, initially static but then tracked if the subject moves.
Focus your camera with Live View
Focusing isn’t always about in-camera calculations. The Live View revolution enables you to gain pinpoint accuracy by using a magnified view of the subject.
In low-contrast situations – when shooting into the sun, for example – AF can be inaccurate.
Live View can minimise inconsistencies or the irritation of ‘hunting’, where the camera searches the focal range while attempting focus lock.
Three simple ways to ensure spot-on focus
Choose your focus mode
Think about your subject and set the focusing mode to suit. If the subject is constantly moving, even by a small amount, select continuous focusing (AI Focus). When working with limited depths of field, critical focus is absolutely crucial to image success.
Select your focus point
Think about your subject matter and then select the most appropriate focal point. Is the subject in the centre of the frame or off to one side? An inaccurately positioned focal point will ruin the shot, so think about this before raising the camera to your eye.
Don’t forget Live View
For static subjects, Live View is by far the best way to ensure focus lock. Set your lens to Manual Focus (MF), focus and then refocus, ensuring the screen really snaps into critical sharpness before tightening down the tripod for extra stability.
Best camera focus techniques: 10 surefire ways to get sharp photos
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Getting sharp images: every photo technique you need to know starting out
Manual focus: what you need to know to achieve pin-sharp images