Most autofocus systems work by detecting contrast in things, so they’ll struggle when faced with featureless things – a blue sky, a plain wall etc. You’ll need to be aware of this when you’re trying to get things in focus – you might need to do the old “reframe/recompose” trick. Contrast is also lower when there’s not much light around, and since AF works via light, you may find your AF struggles when there’s not much light around. Don’t forget that your AF isn’t just working on the light that you can see, but the light that gets through to the camera, so if your lens has a slow maximum aperture, and you’re working in very low light, you may find your AF really struggles. Another reason I like shooting with fast primes!
Some cameras, and many flashguns, have an “AF-Assist” function, which is generally used in low light, and works by simply projecting light into the path of the AF so it can work easier.
Getting everything sharp
If your images are not in focus, there could be many reasons. If the entire image is out of focus then it’s likely that either a) you were focused on something completely different when you pressed the shutter – this could be because you’d left it on manual, or because the AF was aiming for the wrong bit/searching an area of low contrast like sky, or b) you’ve got “camera shake” rather than actually being out of focus. “Camera shake” is when you’ve moved the camera during the exposure, and it gets worse the slower the shutter speed gets. You can solve it by either increasing your shutter speed (assuming you have enough light to get a matching aperture/ISO combination), using a tripod or other support, or using a camera or lens with image stabilisation (IS).
IS/VR (Vibration Reduction) is a very clever little thing that uses gyroscopic motors inside the lens or camera body that counteract the movement you make during exposure. It can give you up to 3 more stops to play with than you would be allowed usually. There is a big catch however – IS is obviously only working on the lens or camera, not your subject. Just because you’re able to use IS to hand hold and get a sharp image at, say 1/30s, doesn’t mean that the sprinter you’re photographing will come out sharp! If the movement you’re shooting is anything other than a snail, you’ll need a fast shutter speed to freeze it, and that’s that.
The general rule to avoid camera shake is that you need to be using a shutter speed AT LEAST as fast as the focal length of your lens – so for a 50mm lens, at least 1/60s, for a 200mm lens, at least 1/250s and so on. I’d stress the “AT LEAST” part of this, as you’ll be surprised how much shake you’ll pick up even with quite fast speeds.
If, however, some of the image is in focus but not the part you’d hoped for, then you need to practice! You might just be using the wrong focus point, which is pretty easy to solve. Lots of people love the beautiful, shallow depth of field you get with DSLRs and bigger cameras, but the catch is that you need to be very careful about your focus, otherwise all you’ll get will be the out of focus bits! Don’t forget – the wider the aperture, the closer you are to the subject, and the longer the lens, the less depth of field you’ll have to play with. This might be exactly what you want, but don’t be surprised if you JUST miss someone’s eyes, or don’t quite get a petal in focus when doing a close up shot. Practice a lot, get familiar with your equipment, and when shooting stuff with a shallow depth of field, be prepared to shoot around the subject a lot.
Different types of “soft” image.
Here are the different causes and symptoms of images not being sharp. To begin with, here’s the image as it’s supposed to be:
Note the shallow depth of field, and that the focus is on the “150mm” part of the lens. Now here’s what the image looks like if we’re out of focus:
Note how the entire image is soft right across the frame. This is usually caused by the camera focusing on the wrong part of the frame, or leaving the camera on manual focus set to the wrong distance. Now let’s see what happens when we’re in focus, but on the wrong part of the frame:
Notice how the vertical rail is in focus, but the “150mm” is now soft. With a shallow depth of field like this, it’s easy for this to happen simply because you’ve got such little depth of field to play with. It can also happen when you’ve got the wrong focus point selected. Finally, let’s see what an image with camera shake looks like:
Look closely at the “150mm” and it’s obvious that the image is actually in focus around that spot, but that the whole image has moved – you can see the “ghosting” caused by the movement of the camera.