In order to study effect of optical zoom and scaling algorithms on image resolution, firstly, lens resolution, detector resolution and photographic resolution are discussed thoroughly. Then, differences between optical zoom and scaling algorithms are deduced from sampling theory. Finally, a resolution measurement system is established, and image of drones are obtained based on variable focus and scaling factors. Experimental results indicate that drone of 100 mm focus is vivid in edge and owns higher resolution, and drone of 50 mm amplified by one time is blurred in edge and owns lower resolution.