Summary: We attempted to accurately predict the focal length for four lens systems: a 100mm lens, a 200mm lens, a of compound objectives and an unknown objective. . This was achieved by using an optical track with mounted light and displayed image, and measuring the location of the image in full focus after the movement of a white display plate installed after the lens system . The best value of f100 was determined to be from unweighted calculations, with a value of f100 = (0.096 ± 0.0004) m, with a percentage error of 3.6%, a fractional error of 0.4% and a deviation of 9.1σ. The best value of f200 was determined from weighted calculations, giving a value of f200 = (0.206 ± 0.001) m, with a percentage error of 3.0%, a fractional error of 0.65% and a deviation of 4.5σ. The compound lens system gave a final image distance of 0.093 m for a separation of 0.1 m and a final image distance of 0.048 m for a separation of 0.2 m. The error percentages were 0.3% and 8.0%, respectively. The unknown lens had a measured focal length of 0.14 m, an error of 7.7% from the known value of 0.13 m. Systematic error within each of the experimental procedures could arise from subjectivity in the focus of the displayed image or damage to the lens in the form of scratches. However, the accuracy of experimental measurements indicates that the manufacturer or lenses may have misreported the focal length of the lens, indicating that experimental values ​​of focal lengths are more reliable than known values. Procedures used: The procedures were followed in accordance with those described in the laboratory handout PHYS-210, for experiment V: thin lenses. No meaningful inferences were made from the laboratory document. However, for the determination of the focal point of ...... middle of paper ...... e, but not accurate according to the known value, causing a high deviation.2. I think our measurements, at least the statistically calculated ones, are much more accurate than the manufacturer's stated focal lengths. From our experimental results, we find that there is a significantly small standard deviation of the mean for the two converging lenses (0.0004 m and 0.0005 m, respectively). Furthermore, the two fractional errors are very small, indicating that the random error accounted for only 0.4% and 0.2% of the average value in each experimental procedure. Therefore, either there was an extremely stable systematic error or the manufacturer limited the quality control of its lenses. Since there were no obvious systematic physical errors in the experimental procedure, we can conclude that the focal point estimated by the manufacturer was probably wrong..