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Part 7 : Introducing auto focus

*Disclaimer: Notes written by Alistair Keddie.


Introducing the DSLR auto focus ability, photography notes



Part 7 : Auto Focus

an example of what the auto focus function may look like on your DSLR camera

By default, usually all of the points in the camera’s auto focus system are on and attempting to lock onto something at one or more of the points. This sophisticated system works well for most of the time, however there are situations where the camera wont lock and refuses to fire. A number of things can cause this from combinations of low light or too many things going on in the scene which confuses the system. Its useful then to be able to over ride this default setting by selecting a single autofocus point, usually the centre one. We do this by accessing the camera’s AF Point selector and switching between each specific point or area. Camera system’s may vary but each will usually allow you to do this. For instance, on the Canon, we access this by pressing the AF Point button then using the selector dial to choose a single point.


focus points on the screen of a DSLR Camera


AF Modes

Like the light meter, the auto focus system also has a number of different modes designed to cope with different situations. Generally there are three of them though like all things across different camera brands, the names keep changing and new ones appear. However, the standard modes usually equate to the following.


One Shot mode (or equivalent) allows us to focus on our subject and then move the camera to recompose our shot. As long as the shutter button is held half way down, the auto focus system will stay locked onto what we initially focussed it on. One Shot is for photographing mainly static subjects.


a static photograph using one shot focus mode


AI Servo mode (or equivalent) is for shooting subjects in motion. It is a predictive mode that will attempt to track anything it has locked onto. For instance, you would use this mode when shooting a moving car or a running child as the camera will attempt to track the subject and keep it in focus. AI Servo is in effect, always on as long as the button is half pressed down and you’ll hear it working as you pan around. In this respect it can be heavy on your battery if expecting to use over a longer period. It works extremely well combined with the centre autofocus point selected allowing us to target our subject. AI Servo is for photographing subjects in motion.


a photograph using ai servo mode to shoot objects in motion


AI Focus (or equivalent) is a kind of half way house between One Shot and AI Servo. Its designed to allow you to lock the focus but if the subject moves, it will then attempt to track and keeps it in focus. It can is useful for subjects where you are not expecting much movement, for instance, on a windy day when the leaves you’ve focussed on shifts in the breeze. AI Focus is for photographing mainly static subjects which might move.

photograph taken using ai focus mode to take a photograph of objects which move

A photograph taken using ai focus mode to take a photograph of objects that stay still whilst the environment moves


Switching to Manual Focus

You can use the autofocus as in the example above to focus on a point in your scene then switch the camera to manual focus. That way you control the focus point of the scene but also lock it to that focal plane. Playing around here with aperture to affect depth of field but also looking to keep the shutter speed fast enough to capture the detail in the waves, particularly the spray as it billows back from the wave in focus.





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