Dr Francisco Belda Maruenda analysed the physiology of the human eye to clarify if it is able to process all the visual information needed to apply the rule.
To apply the offside rule correctly, the referee must be able to keep at least five moving objects in his visual field at the same time (two players of the attacking team, the last two players of the defending team, and the ball).
This is beyond the capacity of the human eye, especially as these five objects can be anywhere within the defenders' half of the pitch, an area of at least 3200m2, says the author. This may explain at least some of the instances when television replays of a game clearly show that the offside rule was not properly implemented.
The relative position of four players and the ball cannot be assessed simultaneously by a referee, and unavoidable errors will be made in the attempt. The use of modern technology during games, such as freeze frame television and frame by frame analysis is advisable to limit these errors, he concludes.