The purpose of selecting focus window is not only to reduce computation, but also to improve image sharpness of object(s) of interest. A simple geometrical model is built using thin lens Gauss equation. The necessity of utilizing different focus window selecting strategies is demonstrated with the model. Then a dynamic focus window selecting method is described. It's reasonable to assume that a photographer's gaze direction points to the object(s) of his interest when he is taking a picture. The gaze direction is trackable by various pupil-tracking methods. A simple modification of viewfinder allows us to take images of photographer's eye with the built-in image sensor. The images are then processed to determine the photographer's gaze direction. One small area in the target image is matched to the gaze direction and selected as focus window. Within the focus window, an uneven sampling method can be used to reduce the computational need further. The uneven sampling works the same way as the human retina. This dynamic focus window selecting method can greatly increase the probability of getting sharply focused object(s) of interest, at the same time only less than 1% of the target image is needed to apply the focus measure.