Here's a page that discusses various types of acuity/resolution measures:
http://webvision.med.utah.edu/book/p...visual-acuity/
The need for imaging systems to be much better than the eye in total due to the fact that the fixation point of the viewer is unknown was expounded by the researchers at RCA in the late 40s and early 50s.
And it's not a fixed number either, as the human visual system adjusts its resolution according to ambient illlumination, whereas the display is always at the same nominal brightness, so that this adjustment does not take place in a TV viewer's eye. Thus, although your visual receptor response gets noisy at low light levels (like TV "snow"), your visual system applies just enough filtering to suppress the noise. But put a TV camera in the same situation, its picture gets noisy, and then the noise is amplified along with the brightness of the scene when displayed on the monitor, and you will see it. Net result is that the camera must do much better than the eye to present an acceptable picture.