  Seeing-limited resolution in large telescopes working over wide wavelength
range depends substantially on the turbulence outer scale and cannot be
adequately described by one "seeing" value. We attempt to clarify frequent
confusions on this matter. We study the effects of finite turbulence outer
scale and partial adaptive corrections by means of analytical calculations and
numerical simulations. If a von Karman turbulence model is adopted, a simple
approximate formula captures the dependence of atmospheric long-exposure
resolution on the outer scale over the entire practically interesting range of
telescope diameters and wavelengths. In the infrared (IR), the difference with
the standard Kolmogorov seeing formula can exceed a factor of two. We find that
low-order adaptive turbulence correction produces residual wave-fronts with
effectively small outer scale, so even very low compensation order leads to a
substantial improvement in resolution over seeing, compared to the standard
theory. Seeing-limited resolution of large telescopes, especially in the IR, is
currently under-estimated by not accounting for the outer scale. On the other
hand, adaptive-optics systems designed for diffraction-limited imaging in the
IR can improve the resolution in the visible by as much as two times.
