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BY ZIV HAPARNAS

In many ways, passive autofocus imitates the way we focus manually. The 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 camera then analyzes the captured picture seen through those regions.

The camera has a built-in computer chip that can run 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 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.

The camera’s goal is to maximize the Focus Level number to achieve 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 constantly 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 camera compares the Focus Level number to a predetermined threshold. If it is higher, the camera announces a successful focus (usually by coloring the focused areas in green). If it is lower, the camera announces a failure (usually by coloring the non-focused areas in red).

The passive autofocus method is relatively cheap to implement as it does not require extra sensors (such as distance sensors for active focus solutions). However passive autofocus 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.

Ziv Haparnas is a technology veteran and writes about practical technology and science issues. He write for printRates, a site dedicated to photo printing, where you can find more information about photo album printing and photography in general.