1. A method of aerodynamic balancing a plurality of wind turbines in a wind farm, each wind turbine comprising a respective tower and a nacelle mounted to the top of the respective tower, a respective rotor rotatably connected to the nacelle for rotating about a rotor axis and having a plurality of equally spaced blades where each blade has an adjustable angle of attack, where rotation of each rotor around an axis generates a value of torsional vibration of the rotor on the respective wind turbine the method comprising: for each of at least some of the wind turbines, measuring the value of the torsional vibration of the respective rotor of the wind turbine; after the value of the torsional vibration of each of at least some of the wind turbines is measured, selecting a plurality of the turbines having a value of torsional vibration beyond a predetermined set value for analysis of the angle of attack of the blades of the selected turbines, thereby avoiding analyzing all of the blades of all of the rotors; for each turbine selected, measuring said adjustable angle of attack of each of the blades of the turbine and in the event that said adjustable angle of attack of one of the blades is different from that of the other blades by an angle difference greater than a predetermined angle, carrying out an adjustment of said adjustable angle of attack of said one of the blades to reduce the blade angle difference; and re-measuring the torsional vibration of the system to ensure that it is reduced by the adjustment, wherein said value of torsional vibration is measured using two vibration sensors at positions mirrored exactly in distance to the left and right of the rotor axis and detecting vibration in a direction longitudinal of said axis so as to eliminate by subtraction all vibration in said longitudinal direction and doubling torsional vibrations; wherein the angle of attack of the blades is measured by the step of: providing a camera for obtaining images of the blades; prior to obtaining images of the blades, obtaining at least one image taken by the camera of a calibration grid; carrying out a comparison of said at least one image of the calibration grid relative to the calibration grid itself to obtain data relating to distortion caused by the camera; positioning the camera below the rotor; rotating the rotor so that each blade in turn is in a reference position in which a tip of the blade is directed toward the camera; capturing an image of the tip of each blade in turn in the reference position; in the captured image correcting angles in the captured image at different locations in the captured image in dependence upon said data relating to distortion by the camera to obtain corrected angles; and carrying out an analysis of the corrected angles in the captured image to determine a blade angle of each blade.