if someone hands you a lens, how could you measure its focal length? use a ruler to measure the diameter of the lens and then divide by 2. use the lens to focus on an object and then measure the distance between the lens and the object. this distance is the focal length. use a ruler and measure the curvature of the lens surface. use the lens to focus on an object and then measure the distance between the lens and the image. this distance is the focal length. use the lens to focus on an object that is very far away and then measure the distance between the lens and the image. this distance is the focal length. hand the lens to your lab partner and ask them to measure it. use to find the focal length.