The optical transfer function (OTF) of the atmosphere has been measured over the ocean for four wavelengths from visible to far IR, using laser sources and a slit scanning telescope. The effects of diffraction, finite slit width, and aberrations have been removed by digital Fourier processing. The shapes of the curves of long term average OTF, and the image-centered (tracked) OTF, as well as the magnitude of the wander variance, all agree well with a theoretical model by Fried. Comparisons of the path-integrated values of Cn2, obtained from the OTF, with the path-integrated values of Cn obtained from CT2, indicate that, for nonuniform Cn2, the weighting of Cn2 as a function of position on the path, behaves as predicted. This weighting heavily emphasizes the part of the path nearest the telescope for imaging devices, whereas scintillation emphasizes the path center. The weighting that applies to imaging devices also applies to beam-forming or projection sys-tems, with the heavily emphasized part near the projection optics. Measurement with a scanning telescope thus yields directly the properly weighted value for such systems.