A method for generating, substantially in real time, a user perceptible indication of the degree to which the distance between a handheld imaging device and a target object approximates the in-focus distance therebetween. A stored image of the target object is sampled, in accordance...http://www.google.com.tw/patents/US5773810?utm_source=gb-gplus-share專利 US5773810 - Method for generating real time degree of focus signal for handheld imaging device 
