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Understanding Autofocus - ePHOTOzine take a look at Autofocus and explain exactly what it is.
What is Autofocus?Autofocus will adjust the lens of your camera to focus on the subject you're photographing to ensure it's sharp.
How it worksWhen you're using autofocus you can use single-shot autofocus or continuous autofocus. Single-shot autofocus is for objects/subjects that are still. When you partially press the button that takes the photograph (shutter button) autofocus will lock onto your subject and stay focused on them. Then press the shutter button all the way down to take a sharp, in focus photo. This happens as half pressing the shutter ensured that focus was locked before you took the photograph.
|Auto works fine for still shots.||But it doesn't do so well with movement.|
Continuous autofocusAs the above image shows, if you're trying to capture a moving subject, single-shot autofocus doesn't work so you should opt for continuous autofocus. This mode allows the focus to continuously change, which means you can track a subject or object that's moving. On the Nikon 1 camera, for example, it can deliver full-resolution images at up to 60 frames per second. Then when you want to focus tightly on one subject, the camera can track them and capture full-resolution images at 10 frames per second.
Continuous autofocus works by predicting where the subject will be slightly before it gets there so when you press the button to take the photograph, the camera is already focused on the right place.
You can also try pre-focusing on a spot you know your subject, say a car on a track, will move through. You will need to lock your focus after you've set it up and be ready to hit the shutter button when your subject moves into frame. Just remember to take shutter lag into consideration so you don't end up with an empty scene. What's shutter lag? Well it's the time delay that occurs from when you press the shutter button to when the photo is actually recorded. The lag time will differ on all camera models but no matter what model you use, it usually means you have hit the shutter button slightly before your moving subject comes into frame, giving the camera time to catch up before your subject is in the correct spot.
When shooting fast movement, having a camera with a fast autofocus system, such as the Nikon 1 camera, which is equipped with the fastest autofocus system in the world*, will help you ensure your subject is always in focus.
|A moving subject will be blured without continous focusing.
||Continuous autofocus is needed for shots like this when someone's walking.|
Sometimes when you take a photo where your subject is off-centre it can confuse your camera and it will focus on the background rather than your subject and as a result, you end up with a lovely sharp background but you can't see your subject because they're blurred. However, there is a way to fix this and it's with focus lock.
This means you half press the shutter button while pointing the camera at your subject then with the shutter button still half pressed, adjust the camera's position so your subject is now off-centre then fully press the shutter button to take your shot.
Tip - Some cameras lock the exposure at the same time as focus which can be problematic when working with lighting that various across your shot. For example, when you're shooting a photo with your subject slightly off centre, it could be darker where your subject is when compared with the rest of the shot. Many cameras have AEL (Auto Exposure Lock) and AFL (Auto Focus Lock) options so you can select focus lock and/or exposure lock.
In cameraFocusing in most modern DSLRs works by using a number of sensors. The more advance the camera, the more AF sensors there are available. Some DSLRs also let you select just one AF focusing zone – many experienced photographers focus just using the central focusing point.
Many cameras now also feature a face recognition mode. This is where the camera finds a face in the scene and automatically focuses, adjusts the flash output and optimises exposure. It's useful for when you want to focus on someone in a group, are photographing someone from a distance or are trying to capture a person while they're moving.
Ocassionally you may find that auto focus struggles to lock focus and if this happens, you may need to switch to manual focus. This is a subject we'll be covering later in the month so keep an eye out for it.
*The world's shortest shooting time lag (as determined by Nikon performance tests). Measured when using the single-point AF mode and the shortest focal length of a standard zoom lens (1 NIKKOR VR 10-30mm f/3.5-5.6). As of August 5, 2011.
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