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For visual observing the Dawes limit (based on a minimum contrast difference between two close Airy disks) is appropriate.
The seeing conditions - the distortions and movements of the stellar image are somewhat overcome by the ability of the eye to "catch" those fleeting moments when the atmosphere is steady.
Imaging generally doesn't have that luxury and just records the photon where they fall on the CCD. The quality of the imaging system defines the point spread function (PSF) - this is if you like the "translation" of the perfect Airy disk to real life conditions. The PSF always degrades the image. The seeing disk (PSF) for imaging is usually quoted as the FWHM (full width at half the peak intensity) - all unsaturated star images will have the same FWHM. The average conditions as per previous threads on the subject can vary from 1.5 to 4 arc sec. This is a much greater image size than that recorded visually.
(This is one of the reasons that the solar imagers use very fast frame rate mono cameras - to try to freeze those "good" seeing moments. It's not unusual to only end up stacking 100-200 frames from an 2000 frame AVI - those being the best quality.)
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