A measurement system was developed using micro-distance imaging technology to measure droplet sizes in the field. Legible droplet images were acquired with a single-lens reflex camera, a micro-distance lens and illumination source in the proper parameters. The droplets were extracted effectively based on the background subtraction, region-growing repair, shape-factor screening and concave points matching methods. The droplet characterizations, including projective area, diameter and volume were calculated using MATLAB image processing technology. The system displayed droplet sizes and distribution regulation in graphs that were convenient for obtaining experimental regulation. Indoor comparison experiments between the developed system and oil disk method was conducted, and the average relative errors were 6.03%, 5.50% and 6.25% for D-V (0.1), D-V 0.5, D-V 0.9, respectively. Field comparison experiments revealed that the relative error was 5.61% between the developed system and the oil disk method and 6.88% between the developed system and the laser instrument method. Application experiments in the field were conducted and the result revealed that fine droplets had better penetration than coarse droplets in the tree canopy. The system was simple in structure, had a lower cost compared with some instruments and reduced experimental time compared with the image processing methods that use oil disks. It has a good prospect for use. (C) 2017 Elsevier Ltd. All rights reserved.