An autofocus method using quasi-Gaussian fitting of image sharpness in ultra-high-voltage electron microscopy.

An accurate method using image sharpness to determine the best focusing is proposed for ultra-high-voltage electron microscopy. This method maximizes image sharpness for adjusting the focus. Five images with different defocus values are used to calculate the image sharpness. To obtain the best focus value that produces greatest image sharpness, fitting the quasi-Gaussian function to five image sharpness is a suitable alternative. This method, which maximizes image sharpness, gives better accuracy than the wobbler method for the ultra-high-voltage electron microscope. The focusing area can be selected without moving the field of view, because the focusing area can be selected at almost any area in the image.