One of the most controversial rules in football is the offside rule. A possible explanation is that the human eye and brain are incapable of processing all the necessary visual information to apply the rule.
The eyes move to focus on objects and maintain them within their visual field. In doing so, they perform saccadic movements, smooth pursuit movements and others movements.
Saccadic movements are rapid, brief, conjugate movements (that is, the eyes move as a pair). They occur when the eyes are inspecting an object, when the gaze shifts from one object to another.
To detect an offside position, the human eye must be capable of detecting at least five moving objects at the same time and determining their positions relative to each other: two players of the attacking team, the last two players of the defending team, and the ball.
1. If these objects are not all in the visual field of the Referee, the Assistant Referees, the public at the stadium and the viewers at home, an offside cannot be judged (it is obvious. -but nobody wants it to see- and very typical in soccers match without Assistant Referee).
2. All the players and the ball are within the visual field: the minimum time needed to detect the three players relevant to an offside position is 160 miliseconds.
3. If one of the players is more than 6 metres away from the referee and another is closer than 6 metres, a further accommodation occular is required (greater of a second). An average player runs at a speed of 7.14 metres/second (100 metres in 14 seconds): in 100 milisecond he will move by 71 cm. If he moves in a direction opposite to the defensive player, the relative change in position between the two will be even greater (fig 2). Sometimes, it is evident that a referee has misjudged the position of players and unduly penalised one of the teams, but why does this happen?
To apply the offside rule correctly, the referee should be able to keep in his visual field at least five objects at the same time (two players of the attacking team, the last two players of the defending team, and the ball), and this may not be compatible with the normal eye function, especially as these five objects can be anywhere within the defenders' half of the pitch, an area of at least 3200 m2.
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 key factor in applying this rule correctly is that the player in question must be in the offside position at the exact time when the ball is passed from a team mate, not when the player receives the ball or when the ball is en route between the players.
By reviewing the physiology of the eye movements likely to be involved in assessing an offside position, I have shown that 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—freeze frame television and frame by frame analysis—is advisable to limit these errors. The complete scientific article can see it in: Can the human eye detect an offside position during a football match? I thank Nuria Navarro Zaragoza for her translation of the paper into English and Joaquín Zaragoza Celdrán for producing the figures.