Abstract: Auto-focus is a key technology in aim-tracking system，which assures of obtaining focus-image in real time.And when springing focus becomes a key question of it.If touch off focus system when image in focus，will result in out of focus.This dissertation puts forward a way on judging an image whether in focus by a single frame，and avoiding an error operation when the image in focus to spring focus.Thereby improve the reliability and the precision of tracking.By testing a series of images，prove that the method is reasonable and credibility.

Key words: image-definition, auto-focus, single frame

摘要： 自动调焦是保证目标跟踪系统实时获得清晰图像的重要技术，而何时触发调焦又成为了其中的关键问题。在图像清晰时如果触发调焦，初始的电机盲动会导致图像可能变得更模糊。提出了一种可单帧判断图像是否清晰的方法，消除了图像在清晰情况下却触发调焦的误操作，从而提高了跟踪的可靠性和精度。通过对多组实际图像序列的测试，证明该方法合理可靠。

关键词: 图像清晰度, 自动调焦, 单帧