A method for dynamic focusing is presented that can be performed by a dynamic focusing controller that can receive images from the image capture device, and for each image, determine a border of the particle within the image, and calculate a pixel intensity ratio of the image based on the border of the particle. The dynamic focusing controller can also calculate a median pixel intensity ratio from the pixel intensity ratios for each image, determine a focal distance direction based on the median pixel intensity ratio, calculate a focal distance based on the median pixel intensity ratio when the focal distance direction is positive, and calculate the focal distance based on a median border width when the focal distance direction is negative. The autofocusing controller can then send an instruction to the focusing mechanism to adjust the image capture device by the focal distance in the focal distance direction.