How Auto Focus Works



Auto focus is another important feature on digital cameras that you should be aware of and…much like auto exposure…knowing how auto focus works will help you use it effectively.

On most cameras the focus area is in the center of your monitor and will be indicated by a small square or corner brackets. As you move your camera you may be able to hear the lens re-focusing.

With many cameras you will be able to see the difference on your monitor or in the viewfinder. It is a good idea to experiment with your specific camera to become familiar with this.

Understanding Auto focus

Let’s begin with a very basic understanding of how auto focusing works. A lens can only record one single plane in critically sharp focus. As objects get further away from this plane they become less and less sharp.

For example…lets say you focus on a subject that is 10 feet from the lens. Everything that is exactly 10 feet away will be in critical focus. Objects less than or more than 10 feet from the camera will not be critically sharp.

Fortunately, however, there is an area both closer to and further from this critically sharp plane that is still acceptably sharp. This is known as the Depth of Field.

I’ll go into this much more fully in the advanced pages, but for now you need only to understand that the sharpest area in your photo will depend on where your lens has focused.

How To Use Auto Focus

Focus Lock Center When you use auto focus, you need to place your subject in the focus area used by your camera, which is often the center of your viewfinder.

However, always placing your main subject dead center in the picture leads to lots of dull photos. Being able to lock in the focus and move the camera for better composition is a much better solution.
In these 3 photos, the green square shows the area that was used for focus.

  • Image #1: Focus was in the center of the composition.
Focus Lock Front Focus Lock Rear
  • Image #2: I moved the camera to focus on the front object, used focus lock and recomposed the shot. Note that the front image is now sharp.
  • Image #3: The focus was on the rear bottle. Then, with the focus locked, I again moved the camera to recompose the shot, but the rear item remained sharp.

As you can see from these illustrations, focus lock is a useful option which allows you to control the area of sharpest focus in your photos, regardless of where in the composition that area is located.

You can consult your owner’s manual to see if your camera has this option, or you can simply do the following assignment…

Assignment: Using Focus Lock

  1. First center your subject in the viewfinder and press the shutter release button halfway down. This will lock in the focus and exposure.
  2. Now, continue to hold the shutter release halfway and recompose your shot so the main subject is off center.
  3. Take the shot and check your result. If your camera has focus lock, the subject will still be sharply focused, even though you moved the camera.

Generally, auto focus will give you very acceptable results. It is mainly with close-ups or when you want to make larger prints of your photos that precise control of the focus point becomes an issue.

As we get into the more advanced assignments, you will learn when and if you want to have more control over focus for the kind of pictures you like to take.

Being able to control focus creatively is one way of turning ordinary snapshots into exciting and dramatic photos.

Here’s to better photography…

Al Hannigan

Al Hannigan


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