Digital projection display has become the mainstream in modern display field. Accurate focusing of the array pixel devices in these systems determines the quality of the displaying effect. Auto-focusing of the pixel devices is the key technique to rise the productivity. Based on the images captured by CCD (Charge Coupled Device) in array pixel device projector, several sharpness evaluation functions, such as gradient operator, Laplacian operator, Sobel operator, Prewitt operator, variance method, and entropy method, are analyzed. Using these evaluation functions, automatic focusing is realized. The characteristics and the auto-focusing result is compared with each other. The experiments indicate that good stability and insensitivity to noise and good regulation effect can be obtainted by variance method.