The dimensional field testing activity described in this paper allowed measuring the vertical and transverse dynamic displacement related to the main span (with a dimension equal to 1012 meters) central section of a long-span suspension bridge the 25th of April Bridge (P25A), in Lisbon (Portugal) using an optical system composed by high focal length lens (600 mm), digital image sensor and active infrared LED targets. Maximum values of 0,53 m and 0,29 m were obtained for the vertical and transverse displacements without rail traffic on the P25A. Passenger train circulation on the bridge's lower rail deck increased the vertical displacement up to 1,69 m. The applied measuring system was subjected to an in situ calibration procedure (SI traceable), showing maximum deviations close to 1,3 mm with an expanded measurement uncertainty (in a 95% confidence interval) around 3,0 mm, in a favorable observation scenario (winter season with low vertical thermal gradient in the atmosphere). Field studies of optical phenomena such as atmospheric refraction and beam wandering effect by turbulence were also performed, aiming the validation of developed refraction correction models and the quantification of targets image coordinates dispersions (0,13 pixel in winter season a 0,56 pixel during summer season).