Autofocus

Most modern cameras, particularly digital ones will come as standard with autofocus. (Very cheap cameras, and in general cameras on mobile phones will have a fixed focus which cannot be changed.)

As the name suggests, the autofocus function will automatically focus the lens on what it considers to be the subject of the picture you are trying to take. The result is a sharp image - most of the time. Sometimes the autofocus function of your camera can be confused by various things or may not work as you expect. This can be caused my many things, including the following:

1. The subject of the picture being off centre

If, for example you are taking a picture of someone with a feature in the background you would like to record, such as a building. You may position the person to the side of the viewfinder and take the picture. When you develop the picture, the person is out of focus and the building is sharp. The camera has 'thought' that your subject was the building and not the person (with the building in the background).

Most cameras have a way around this, called focus lock. To correctly take this picture, you need to point the camera initially at the person then press the shutter down half way only. This will 'lock' the focus of the camera. Next, you move the camera so the elements of your picture are where you want them to be and press the shutter down fully to take the picture.

2. The subject having very low contrast or soft edges

You may be taking a picture of a white rose, against a pale background. The focusing system will find it difficult to pick up on the edges of the image that it would normally use to determine whether an image is sharp or not. This will result in the camera 'hunting' for focus and not being able to find it.

There are various ways around this problem.

First of all, the camera may have a 'focus assist lamp' which is usually red (your manual will tell you whether your particular camera has this, or not). This lamp will automatically fire as the camera sees fit to try and improve the illumination of the image. Alternatively, the camera may use a burst of its own flash to achieve the same thing.

Secondly, your camera may have the option to manually focus the lens. This can normally be achieved through the menu system of the camera and while manually focussing, the camera will increase the magnification of the image to help you see if it is sharp or not. This method can be hit-and-miss with most cameras, unless you are using an SLR style camera.

The final method involves helping the camera out. You can either increase the illumination or move the source. Alternatively, place a more defined object within the frame where you would like the camera to focus and use this along with the focus lock (press the shutter down halfway). Once the camera has focussed, remove the object from the frame and press the shutter down fully to take the picture.

3. The subject glistening (like the sun on water, or snow) or being highly reflective

If you are trying to take a picture of swans on a lake, a beach scene or a picture of someone skiing; the high reflectance of the surface can cause the autofocus to fail. The camera is being confused by the spots of very bright light reflected off the surface.

If your camera has a filter attachment (screw thread) on the front, you can buy a polarising filter to cut out the high reflectance from the surface. Make sure it is a circular polarising filter if your camera has autofocus, as linear polarising filters will confuse the camera's systems further. Alternatively, try changing where you shoot from (i.e. where the sun is in relation to your camera). Moving by a small amount may be enough to allow the camera to focus correctly.

4. There may be something in between the camera and the subject

When trying to take a picture of an animal at a zoo, or a picture of something through some trees the resulting picture may be blurred. This is because the camera has focussed on the obstruction between the subject and the camera. With the above examples, the animal at the zoo may be behind some chain link fencing or glass, while the trees may come into the focussing range of the camera.

Solving this issue can be tricky. The first thing to try would be to move the camera slightly while half depressing the shutter repeatedly. You may find a point where the camera is happy to focus on your subject. If this is unsuccessful, you may need to switch the camera to manual focussing mode and focus on the subject yourself. With glass, it may be possible that you can clean it, and the camera won't 'see' it. A better solution though is to get as close to the glass as you can, and put something like a jacket over you and the camera (forming a hood) to cut out all reflections on the glass. This should allow the camera to focus straight through it.

5. The subject may be too close to the camera

Every camera has a minimum focal length. That is the minimum distance that an object must be from the camera, before it can be focussed on. If the camera has a zoom lens, then the minimum focal length will vary depending on how much zoom you are using. With most cameras the minimum focal length increases as you zoom further.

The obvious solution to this problem is to move further from the thing you are trying to take a picture of. But, perhaps this is not ideal as you would like to fill the frame with the object. Most digital cameras have a way of getting around this problem though, called a 'macro' function. This function is generally symbolised by a small plant. Your camera may have a dedicated button that can be pressed or a dial that can be rotated to activate it. If not, then the menu system within your camera will allow you to access it. This will then allow you to move much closer to the object and still focus successfully. Some modern digital cameras allow you to focus on objects as close as 1cm from the lens. Remember to press the button again once you have finished with 'macro' to return to the normal focal length.

6. There may not be enough light

Autofocus systems rely on light. The camera has to be able to 'see' what you are trying to photograph to focus on it automatically. It follows that if there is not enough light, then the camera will not be able to successfully autofocus. In the last few years, this has become less of a problem as digital cameras have improved, but can sometimes be an annoyance. The most obvious solution to this problem would be to switch to the manual focussing system of the camera and focus on the subject yourself.

If your camera has an 'Autofocus assist' function, this might not be a problem. If the camera struggles to focus due to low light, it will fire a dedicated beam of light into the scene to help with focus. This is usually a red light, although sometimes the camera uses a strobe burst of flash to achieve the same thing.

7. The subject may be moving too fast for the camera's autofocus system

When photographing a fast moving object like a motorbike on a racetrack, the camera may not be able to adjust the autofocus quickly enough to capture a sharp image.

Some cameras have a 'sports' option that will change the autofocus from a 'once' mode to 'continuous'. This should more successfully track the movement of an object, but only works to a point and will increase battery drain as the camera's optics are being constantly moved. A more favoured way of overcoming this problem is to pre-focus the camera on a point where you would like to take the picture and wait for the object to reach that point. You do this by literally pointing the camera at the place you want to take the picture and 'locking the focus' by pressing the shutter halfway down. Now you must keep the shutter pressed halfway down and when the object reaches the correct point, press the shutter fully.

You may need to 'pan' the camera while doing this to get the subject captured sharply. Once you have 'focus locked' the camera as described above, you should follow the movement of the subject (panning the camera) until it reaches the point that you used to 'focus lock' before pressing the shutter fully. This technique takes practice, and while the delay in digital cameras (between pressing the shutter, and the camera actually recording an image) has decreased dramatically in recent years, this should still be factored into the equation.

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