Auto Focus: Blessing or curse?

photo by Ray Tharaldson all rights reserved 2010

5 Reasons to Turn off Your Autofocus

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?

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.