Specification of vibration modes, amplitudes, and damping coefficients of structures are crucial issues in civil and mechanical engineering. Several techniques have been used for this kind of studies, including holographic interferometry, speckle interferometry, and moiré technique. But, for a large-scale structure, the modal analysis technique is usually used. We use the time-averaged digital image of a sinusoidal pattern fixed on a vibrating 6-m iron I-beam to study in-plane vibration. The study includes specification of vibrating modes, amplitudes, and damping coefficients. Using a wide-angle high-resolution digital camera, successive images of the vibrating pattern are recorded in exposure times much longer than the vibration period and much shorter than the relaxation time. The visibility measurement along the images leads to the specification of the mentioned parameters.