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Thursday, 24 December 2009

10 Reasons to Turn off Your Autofocus

A Guest Post by Alistair Scott.

When I started using a camera autofocus was something out of science fiction. I mean … it would never work in real life, would it? Apart from anything else, how could it know what you wanted to focus on?

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Now fiction has become fact, and pretty well every camera has AF as standard. It works, and works well. But it doesn’t always work perfectly. It can pick up the wrong thing or fail to find anything to focus on, causing the lens to ‘hunt’ back and forth. Sometimes it won’t even let you fire the shutter.

So, here are ten situations when it’s worth turning your autofocus off and going back to the ‘good old days’ of manual focusing:

1. When there’s not enough light

In low light, contrast is also low, and AF relies on light and contrast to latch on to things. Your camera may have an AF assist lamp built in. But, even if you have it switched on, it won’t work in situations like in the shot above.

Though the image looks bright enough, in reality there was little light, and it required a 30-second exposure.

2. When there’s not enough contrast

If your AF metering spot is on something like a plain-coloured wall the camera will find it impossible to focus, no matter how bright the light, and the lens will ‘hunt’. You can re-frame your shot temporarily so the spot is on something with detail and press the shutter half down to activate the AF. Then keep the shutter button half-pressed to lock the focus, and go back to your original composition. Or focus by hand.

3. Shooting wildlife

Most wild animals have excellent hearing and, no matter how good your autofocus, it will make a noise. Even the slightest whirr is likely to spook wildlife. Switch it off if you want those great, natural wildlife shots.

4. With landscapes

When shooting landscapes you usually want things in focus from the foreground to distant mountains. This means closing down the aperture to increase depth of field and focusing about a third of the way into the scene (at a point called the ‘hyperfocal distance’ where everything from quite close to infinity is sharp). Switch off the AF. If you leave it on, when you press the shutter it will re-focus … probably on those far-off mountains.

5. If you’re doing HDR

High Dynamic Range photography involves taking several shots of the same scene, all exactly the same except for exposure, then blending them when you’re back at the computer. It’s important to have identical focus in each shot to ensure success. With AF on, it may choose a slightly different focus point for each shot.

6. Fast action

When you’re photographing a fast-moving subject, your AF will have its work cut out to keep up with the changing distances. Most times it fails. At this jump, in a radio-controlled buggy competition, I first started shooting in burst mode, with the AF switched on. This was the sort of thing I kept getting.

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It wasn’t until I switched the AF (and burst mode) off and pre-focused on a spot where most of the buggies landed that I started to get decent shots.

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7. Shooting through glass

Taking photographs through glass is generally not a good idea. Avoid it if you can. But sometimes it’s unavoidable, e.g. if you’re in a plane, or photographing fish in an aquarium. The trouble is, the AF may home in on reflections, or marks on the glass. So switch it off.

8. With portraits

The golden rule of portraiture is to focus on your subject’s eyes. What’s more, you often use a wide aperture to throw the background out of focus. But if your autofocus picks up your subject’s eyebrows or the tip of the nose then, because you’ve got such a narrow depth of field, the eyes will be blurred.

9. Macro

With macro photography autofocus has a hard time. The depth of field is so tiny that the camera has no idea what you want to focus on and the lens is likely to hunt wildly. You definitely must take control.

10. Composition with the ‘Rule of Thirds’

Many cameras have the autofocus spot fixed in the middle of the viewfinder. This means that if you’re composing with your subject at one of the classical ‘thirds’ positions it is likely to be out of focus.

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In this image a centralised autofocus would not pick up the boat and, on top of that, it would have difficulty latching to the smooth water of the lake.

So … autofocus is brilliant but it’s not infallible. A good photographer knows when to take control of the camera to get great shots in challenging situations

Alistair Scott is an award-winning freelance photographer and writer who has travelled the world widely. He lived for 20 years in Africa, but is now based in Switzerland. His latest book is ‘The LowDown Guide to Family Photography’, which can be seen at www.alscotts.com/fampage

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