One method to measure thin film elastic properties is use of the vibrating reed technique to compare stiffness of uncoated and coated cantilever beams. Since the change of frequency is used to calculate the modulus, the change of frequency caused by curvature due to film stress has to be investigated. We present simultaneous measurement of resonant frequencies and curvature to show that curvature can cause large effects, depending on the geometry. An approximate calculation indicates in which cases stress may be neglected. A method to determine stress from combination of torsional and flexural vibrating frequency measurements is proposed.