In focus objects in a digital photo is a very basic requirement for high quality photography. There are two different ways to focus on such objects: manual or automatic. There are a few automatic focus methods one of them is known as passive auto focus.
High quality sharp and crisp digital photos are a result of many optical parameters that need to be set right. One of the most important optical parameters is focus. When objects in a digital photo are out of focus they look blurry and are missing details and clarity. When objects are in focus they look sharp and crisp.
While focus can be set manually by the photographer in most cases using the digital camera’s automatic focus feature is much easier and faster. There are many different algorithms and methods that digital cameras use in order to automatically determine the right focus for a specific scenario. One of those methods is knows as passive auto focus.
Passive auto focus
In many ways the passive auto focus imitates the way in which we set the focus manually. The digital camera defines one or more regions in the picture to focus on. These areas are usually around the center of the photo and are marked as rectangles on the viewfinder or the LCD. The digital camera then analyzes the captured picture seen through those regions.
The digital camera has a built-in computer chip that can run image processing algorithms. The camera executes such image processing algorithms to determine a Focus Level number. The exact way in which such a number is calculated is out of the scope of this article. A very simplistic explanation is that the digital camera transforms the digital image to a frequency space and measures the amount of high frequencies in the photo (high frequency in an image correlates to high contrast or to focus). The more high frequencies present the more in focus an image is and the higher the Focus Level number is.
The digital camera goal is to maximize the Focus Level number. In this way the digital camera achieves the best possible focus (or at least theoretically achieves such a focus). The digital camera does that by moving its lenses back and forth as it recalculates the Focus Level number. The camera is searching for a position where the Focus Level number is the highest.
When such a position is found the digital camera compares the Focus Level number to a predetermined threshold. If it is higher the digital camera announces a successful focus (usually by coloring the focused areas in green). If it is lower the digital camera announces a failure (usually by coloring the non-focused areas in red).
The passive auto focus method is relatively cheap to implement as it does not require extra sensors (such as distance sensors for active focus solutions). However passive auto focus can also fail. The reasons can vary: poor lighting conditions, low contrast objects that are hard to focus on like walls or solid surfaces and others. When the auto focus fails you can either try to focus on other objects in the same distance from the digital camera, lock the focus and pan back to the original objects you wanted to capture or you can revert to old fashion manual focus.
http://www.articleblender.com/Article/How-does-a-digital-camera-passive-auto-focus-work/79005
