How Live View autofocus works: what your camera’s manual doesn’t tell you

Contrast AF vs Phase detection AF: key differences explained

When you switch your camera to Live View, the image is no longer reflected up into the viewfinder or down on to the phase-detection AF sensor. It passes straight through to the sensor, which relays the image ‘live’ to the LCD display on the back of the camera. The image on the sensor is also used to check and adjust the focus. In this tutorial we’ll explain just how your camera’s Live View autofocus works and how you can use it to ensure optimum sharpness.

How Live View autofocus works: what your camera's manual doesn't tell you

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The design of the single lens reflex camera is clever because it enables you to see your subject through the camera lens.

This is achieved with a mirror inside the body which reflects the scene up on to a focusing screen.

This image is reflected into the correct orientation with an optical pentaprism, then viewed through the viewfinder.

At the moment of exposure, the mirror flips up, the image passes through to the back of the camera, and the shutter opens to expose the sensor.

When autofocus systems came along, camera makers needed a way to get the autofocus sensor in the light path while you’re composing the picture, and so they made the main mirror semi-transparent and added a hinged sub-mirror to the back, which reflected some of the light down to the autofocus sensor, mounted in the base of the body.

This sensor checks focus using the principle of ‘phase detection’, which compares two slightly offset images of the same subject and uses the distance between them to work out which way to focus, and how far. It’s fast and efficient.

Adding a Live View mode to digital SLRs gave camera makers another problem. In Live View, the mirror has to be moved out of the way so that the image can pass straight to the sensor.

This means that the sub-mirror can no longer reflect the image to the autofocus sensor.

SEE MORE: Live View – how to use it on any camera

Contrasting approach
The solution was to use ‘contrast autofocus’, as found in compact cameras. It’s based on the fact that an image (or adjacent points on an image) will always have the highest contrast when it’s in focus.

Contrast autofocus involves trial and error. The camera has to change the focus and then see how this affects the contrast.

If it goes up, it shows the camera is moving the focus in the right direction, but it’s just as likely the camera will move the focus the wrong way and then have to backtrack.

That’s not all. The camera won’t know it’s found the focus point until it goes past it and the contrast starts to fall again, at which point it will have to backtrack once more.

SEE MORE: Autofocus point options: what subjects should each be used with

This is why contrast autofocus is slower. It has to make an adjustment, check the result and try again as it zeroes in on the correct focus point.

You probably won’t be aware that all this is going on. Modern contrast autofocus systems are faster and more efficient than early versions, and you may simply be aware that the focus is ‘hunting’ more in Live View mode than when you’re using the viewfinder.

Slow but sure
While contrast autofocus is slower than phase-detection AF, it does have some advantages. For a start, it works across the whole frame.

In Live View you can use the multi-controller to move the focus point anywhere you like, right up to the edges of the picture.

SEE MORE: Master your camera’s autofocus – which AF points to use and when to use them

Contrast autofocus is also better suited to face recognition; the camera can automatically locate and focus on any faces.

Most enthusiasts would probably prefer to pick the focus point manually, but this is a selling point in cameras designed for novices upgrading from a compact camera.

The other thing about contrast AF is that it remains active while you’re shooting movies. The camera can refocus as your subject moves while you’re filming or, if you use the Subject Tracking AF mode on the newer models, it can track it around the frame.

When you switch to Live View you do have to learn new focusing tricks. But once you understand it, Live View autofocus stops looking like the poor relation. It’s slower, but accurate and, in some instances, more effective.

PAGE 1: How Live View autofocus works
PAGE 2: Contrast AF vs Phase detection AF – key differences explained

READ MORE

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