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June 26th, 2008 admin Leave a comment Go to comments

How Does the Camera Auto Focus Work and Why Sometimes it fails?

How does auto-focus work?

Each of us use it every time we click the shutter button. Almost all modern cameras include some sort of an autofocus system. Thanks to the autofocus system, we enjoy an easier photography experience and can concentrate on composing the right photo and capturing the moment instead of manually setting the focus.

Auto focus does have its limitations. For example sometimes one might want to produce pictures that are a bit vague as an artistic expression. The auto focus implementation has its limitations and in some scenarios could fail. An example is the use of a high-end SLR camera with a passive autofocus system for a picture of blue skies. In most cases the camera will move its motor back and forth and will eventually give up and not concentrating.

To better use of the autofocus system would help to understand high it actually works. Although implementations can vary, all we can divide them into two categories: passive and active. Most pocket cameras use the cheaper passive method while high-end professional cameras use the active or a combination of both.

Passive autofocus:

Passive autofocus can be seen as imitation of how we focus manually. The camera is one or more regions in the picture (usually they are marked as rectangles on the viewfinder or LCD screen). The camera analyzes Then the picture seen through those regions and calculates a Focus Level number. The camera then tries its lenses back and forth to move when it recalculates the Focus Level. The camera looks for a position where the Focus Level is the highest. For that point if the Focus Level is above a predetermined threshold of the camera would define this region as the picture in focus.

The Focus Level can be calculated in many ways. The common feature of all calculations is figuring out how much contrast there is in the picture. Although not in the scope of this article a way to calculate such a number is by running the photo through a high frequency filter - this is based on the fact that high contrast is associated with high frequencies.

Active auto focus:

Active autofocus works by measuring the distance between the camera and the object in the picture. Technically if you knew the exact distance to the object you take a picture of you could set the lens focus on the exact position. The active focus system shoots a beam of invisible light, usually infrared, at the object in the middle of the picture and measure the distance to that object. Based on that distance the focus is set.

Combined auto focus:

Some high-end cameras combine both systems. The camera picks the right system for the specific scenario or will cross check and use both at the same moment. The photographer can also decide to hand one of the two options. For example when shooting blue skies the camera can try to use the active system and the distance measure. Since the distance is infinite the camera can determine the focus and skip the passive focus. In other cases where the distance is not infinite the camera can use the active system the lens into an approximately correct position and use the passive system for fine tuning. In the dark scenarios the camera can choose to use it because the active system the passive one will not work.

So why not the autofocus works all the time?

Same with all the electronics and computing in the camera there are always scenarios where the camera auto focus fails. It can not be if the camera can not focus and the image is fuzzy or sometimes when the image is actually in focus, but the camera "thinks" it is not.

What causes such cases? The list is long, but here are just a few examples: - Taking low Featured Photos: The passive auto focus system must "see" the picture and to work in low light scenarios this is not possible. Some systems use a series of flashes to overcome this limitation, but this solution is not much. An active system can measure the distance to the object in such scenarios but will fail If the item is not in the middle of the picture or if there are a few objects at different distances. - Active systems can not with objects that tend to the infrared beam it Using absorb. Some materials absorb infrared rays and is the active system to measure the wrong distance. In some scenarios other infrared sources such as candles and open flame fires the active system useless. - Low contrast objects such as white walls or blue skies. The passive autofocus based on the fact that the Focus Level significant changes when moving the lens back and forth. This allows the camera to focus attention on the correct position. The Focus Level of low contrast objects does not change much and not the passive system.

Knowing how the auto focus system works helps a photographer to understand why sometimes the camera can not focus. In such scenarios, photographer looking for other solutions. Sometimes the photographer will have to use the manual focus. In other cases directed to another object in the image in the same distance, but easier to target and lock the focus on that object will solve the problem.