The customary model, that is, bsin = m, for the measurement of the sizes of slots and thin wires by optical diffraction has been used widely for a long time. In this design note, the model is analysed theoretically and experimentally and improved to the new model, btan = m. Two calibrating slots and thin wires (thin cylinders) are measured by means of optical diffraction. The results show that the new model btan = m is more suitable for describing optical diffraction phenomena and more accurate for measuring a thin wire's diameter.