You might be interested to know that the first commercially available autofocus camera was actually a point and shoot model! Autofocus was developed in the 70s and 80s and revolutionized the way people used their cameras. Photographers today just take autofocus for granted. That is, of course, until it doesn’t work!
It’s probably happened to all of us - you compose your picture, half depress the shutter button to focus... and the lens whirs backwards and forwards (this is called 'hunting') without focusing as we expect it to. For some reason the lens 'hunts' for a focus point but can't find it. Or you take a photograph and find that the background, or foreground, is in focus but the subject isn't. Very irritating! Here's how to avoid the problem.
Note: Where you’ve taken a photograph and nothing is in focus, this is probably caused by camera shake and is not an autofocus problem. Use a tripod or a faster shutter speed (1/100th of a second or quicker) to resolve this issue.
Point and shoot cameras
In very simple terms autofocus systems are either active, or passive, or more complex modern systems. Point and shoot cameras usually employ the active system where an infrared beam is emitted by the camera. This is bounced off the focus subject, back to a sensor in the camera, which then focuses the lens for you.
A camera which relies on an infrared beam to focus can be ‘confused’ by an infrared light source in the scene you are photographing. A candle flame gives off infrared light. This may explain why you’ve had trouble focusing on someone blowing out candles on a birthday cake! By stepping back and including more of the scene in the picture you reduce the size of the flames and this can allow the camera’s autofocus to work properly.
You may also have had problems focusing on a very bright subject (like the hood of a silver car in bright sunshine) or on something near a very bright light – this situation often ‘blinds’ the camera’s sensor to the returning beam making it difficult to focus. You can try to reframe your picture to reduce the glare, or shield the lens from the light source. If this isn’t possible, or doesn’t solve the problem, you’ll have to use manual focus, if you have the option.
If there is something between you and the subject you are trying to focus on (a mesh fence for example) the beam may cause the lens to focus on the intervening object instead of the subject. Moving your body position slight and refocusing may solve this problem.
Some other tips:
<li>Some glass is designed to block infrared light. You’ll find it difficult or impossible to photograph scenes through this kind of glass. Positioning your camera at 90° to the glass might help. </li> <li>An active autofocus system makes it impossible to focus directly on a subject that is not in the centre of the frame (and so is not in the path of the beam). Simply point your camera at the subject, focus by pressing the shutter button half way down, hold the button, recompose your photograph and fully press the shutter to take the picture, with your off centre subject in focus!</li> <li>Some black surfaces absorb so much infrared light that there is not enough left to bounce back to the camera sensor and enable it to focus. Focus instead on something close to the dark surface, recompose and take your photograph.</li> <li>You won’t be able to focus on an object that is out of range of your camera’s infrared beam.</li>
SLR and DSLR cameras
Older SLR cameras usually use a Passive autofocus system. This involves a tiny computer driven system which focuses the camera by analyzing the image seen through the lens. In order to work properly the system needs some contrast in the scene. If there is no detail, for example in a cloudless blue sky, then the camera can’t focus.
This autofocus system seems to work more efficiently where there is some vertical detail, which means that it might be possible to solve your focusing problems by turning your camera from a horizontal to a vertical position. By doing this you reorientate the sensor to the detail in the scene – you obviously can’t turn the scene on its side!
Newer cameras have made things easier for you by building in more horizontal sensors.
Most DSLR cameras now use an even more complex autofocus system, although this still works by analyzing contrast in the scene.
Because your camera needs to ‘see’ some detail or contrast, the autofocus will operate less efficiently under low light conditions. You can ‘help’ your camera by using the centre autofocus point (the most sensitive one) and by focusing on an area of the subject with the most contrast.
The sensor behind the autofocus point in the viewfinder of a DSLR is often wider than the little square that we use to position over the subject on which we want to focus. If we focus on a small detail using an autofocus point but there is stronger detail just outside the point, the lens may focus on the stronger detail instead, or it may ‘hunt’ between the two. This problem is best solved by using manual focus.
Photographing very repetitive patterns may also cause a problem for your autofocus. If you can find something that is the same distance away from you as the patterns, you can focus on that by pressing the shutter release half way down, then recompose your original picture and take the shot. Manual focus is another option.
It’s safe to say that autofocus works effortlessly well the vast majority of times we press the shutter release to focus on a scene! Being able to solve the problem when it does occasionally fail is a matter of understanding why it’s not working, and what (if anything) we can do to correct the situation.
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