Focusing is the process of forming a sharp image on the sensor or film. In the earliest cameras the photographer would replace the plate with a ground glass screen. He would put a dark cloth over his head so he could see the image and focus the lens until the image appeared sharp. In an SLR camera you still have a Focusing screen, which you look at using a viewfinder. A moving mirror is used to automatically switch between the screen and the film or sensor.

Of course, you are used to modern cameras Focusing automatically. Even your phone manages to do a good job of it, but it is really quite difficult

Early DSLRs had only a few Focusing points, which were displayed with little red squares in the viewfinder. Sometimes only some of these worked, depending on the lens in use. And normally the centre point was much better than the others. As time has gone on the number of Focusing points has increased and they have spread out from the centre to cover the whole frame.

But what do we mean by a Focusing point? Not all of an image can be in focus. Only objects the same distance from the sensor can be perfectly sharp. So the camera has to choose what to focus on. It looks at all the Focusing points and simply chooses the point on the nearest thing. It can only do this if the subject has enough contrast to be detectable. Sometimes if the camera can’t focus it will hunt, with the lens moving between the limits of its Focusing range.

The camera can be told to use all of its Focusing points or just a few. Many photographers stick to just the centre one because of its better performance.

You start the camera Focusing by half-pressing the shutter release. If the camera focuses successfully it beeps and display an indicator in the viewfinder. Then you are able to continue pressing to take the picture, Some cameras have a setting that allows you to prioritise taking a picture over achieving focus, An out-of-focus image is sometimes better than none at all.

Once the camera beeps the focus is locked, If you focused by placing the centre point over the subject now is the time to reframe the image to get the composition you want, before taking the picture. This technique is called focus-recompose.

Better cameras have small joysticks that allow you to choose the focus point you want to use. You can select the point over the subject rather than moving the whole camera. This leads to better Focusing accuracy than focus-recompose.

If the subject is moving the problems multiply. As the subject moves across the frame the camera might need to track it as it moves from focus point to focus point. If the subject is moving closer or further away the lens needs to be continuously refocused.

Because of these requirements, most cameras offer two autofocus modes. A single shot mode that takes its time to achieve perfect focus on a static subject. And a continuous focus mode that prioritises speed over accuracy and refocuses as the subject moves closer or further away. Taking a picture of someone walking towards you on a catwalk is an example of when you might need this,

As the computers inside cameras have become more powerful it has become possible for them to detect and recognise faces. This allows you to delegate the choice of focus points to the camera. Some will even focus on an eye, which is normally what you want to be sharp.

All Focusing systems need plenty of light and subject contrast to work well. This seems to be particularly true of face detection systems.

Finally, it’s still possible to focus manually. The autofocus system will normally do a better job, but using manual focus can help you engage with the image in a way that isn’t possible if the camera is doing all the work for you. Most cameras will allow you to focus by turning a ring on the lens. But in modern lenses, this is an electronic control that commands the lens to focus electrically, rather than being directly coupled to the Focusing mechanism. For the real manual focus experience, it is better to use a lens that is only manual focus.

For homework try Focusing on something using normal single-shot mode. Keep the shutter button half-pressed to lock focus, and walk towards the subject. Watch it go out of focus. Now try the same exercise using continuous autofocus. You should be able to hear the lens reFocusing as you move and observe that the image remains sharp. Notice too that the camera doesn’t beep in continuous autofocus.

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