New Zealand D-Photo

THE FINE POINTS OF FOCUS

Get to grips with the increasing­ly advanced focus features being served up by modern camera models

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Focus is one of the most essential elements of photograph­y. It can seem basic — an image that is sharp as opposed to blurry is in focus — but there’s a lot that goes into understand­ing and mastering the process.

Focus is achieved when light passes into a lens, glass elements within the lens are moved to change the optical path of the light, and the resulting image on the sensor will now contain a particular plane of focus. Put more simply, you make sure the subject of your image is in the plane of focus to ensure that it’s sharp and clear.

FOCUS SYSTEMS

There are two primary ways to focus an image: using either autofocus or manual focus. Capable lenses will have a switch on them allowing you to change between the two kinds of focus.

A lens capable of autofocus has a motor inside to adjust the lens elements to automatica­lly get your photo in focus — sometimes this function is handled by the camera body instead of the lens. Autofocus technology is a particular­ly advanced bit of business, but you don’t need to know every intricate in and out to master it. So long as you can differenti­ate between the two main types of autofocus systems, phase detection and contrast detection, you’ll at least know the right technique for any given situation. Phase detection: To find focus in this mode, the camera feeds light into tiny microlense­s above tiny sensors, which take data from the extreme sides of the lens and calculate how much and in which direction to change in order to achieve focus. The splitting of the light is done with a convention­al DSLR’s prism, which is why mirrorless cameras (with no prism) do not offer phase detection.

This autofocus system is resource light and very fast, so it is great for tracking and focusing on fast-moving subjects. It also gives an accurate idea of an image’s depth of field. Low light situations can prove a challenge for phase-detection autofocus and may yield inaccurate results.

Contrast detection: This mode finds its focus without the help of a prism, instead working with light data that have hit the camera sensor directly. Contrast detection works on the assumption that, when a subject is in focus, its contrast will be at its highest. To discover that point, the system pushes the focus point backwards and forwards.

Different brands implement this process in different ways, but, in general, it is a slower process than phase detection, requiring a fair amount of computatio­nal power. If your subject isn’t moving, the slower speed is not a problem and you get the bonus of much more accurate focus results. Both autofocus modes will yield usable results; you only really need to chop and change when presented with particular situations — fast subject, low light, etc.

Manual focus: This does away with all the automation and allows the photograph­er to focus an image by hand. By turning the focus ring on the lens, you are manually adjusting the lens elements to find focus. It’s not a quick method, but it does allow for overriding any errors the autofocus might have run into due to tricky conditions.

FOCUS MODES

With a working understand­ing of autofocus systems, it is now time to choose the right autofocus mode. This is the setting that dictates how the autofocus will behave while you are shooting.

Single-servo: Also called ‘One Shot’, ‘AF-S’, and other variations depending on your brand. This is the most basic forum of autofocus: depress the shutter button halfway and the camera will lock the focus onto the subject you have placed under the active autofocus sensor. This focus will be held until you capture the image or release the shutter and refocus.

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