Seeing is defined as the image size (FWHM) in arcsec on a long exposure limited by the atmosphere (ie. telescope without optical aberrations nor dome/mirror seeing) as observed at zenith and at 0.5 micron.
The seeing varies with the airmass to the power of 3/5th and with the wavelength to the power of -1/5th. The seeing is in principle, as long as the above criteria are fulfilled, independent of telescope diameter
and the measurements by the differential image motion method (DIMM) fit well to all medium size telescope.
For the recent family of diameter 8m and more however, the image quality appears better than predicted by DIMM because of a strong contribution of the surface layer below 30m
(The Messenger no.132-jun08). Secondly, the science images are improving at long wavelength faster than
the standard -0.2 lambda power law dependency (see the
section on outer scale effects).