It's official - football referees really are blind

Football referees are often told they need their eyes examining - but sending them to the optician wouldn't do any good. Scientists have concluded that when judging whether players are offside the human eye is simply not up to the job.

Their argument depends on having a working knowledge of the offside law, itself frequently a matter of bar-room dispute. Law 11 of the International Football Association Board states that a player is offside if he is nearer to the goal than the ball unless there are at least two of his opponents between him and the goal line (or he is in his own team's half).

FIFA recently ruled that if a player is level with the first of the two opponents then he is not offside. The law previously stated that he had to be behind the defender.

As the law stands at the moment the offside judgement should be made not at the moment the player receives the ball but at the moment the ball is passed to him by one of his own side.

It is this aspect of the law that puts it beyond human capabilities. Dr Jaime Sanabria, a neurologist, and colleagues from the Fundacin Jimenez Daz, Madrid and a referee from the Spanish Royal Football Federation have calculated that because of the way the eye focuses, the linesman (now known as the assistant referee) who has to make the judgement, has an "erroneous image of the initial positions of the players at the exact moment the pass of the ball began."

The reason is that the eye makes adjustments, called saccadic eye movements, to guide visual images on to the central part of the retina that "sees" best, known as the fovea.

In their paper, published in the The Lancet, Dr Sanabria and his team described defenders springing an off-side "trap" on an attacking striker and the difficulty this posed for the linesman.

One situation envisaged the last defender and attacking player in the same line but then, as the pass is made, starting to run in opposite directions at about five metres per second.

These movements can take 250 to 300 milliseconds to complete and in that time an attacker and defender who started level on the pitch may be four or five metres apart. To make an offside judgement, the assistant referee has to fix the fovea on the last defender (assuming, as is normal, that the second defender is the goalkeeper) and then assess the position of the attacker about to receive the ball - a difficult task given the time required to focus the image.

For armchair referees with the benefit of a slow-motion replay the gap between the players may be obvious but on the field it will look different.

However, the authors point out that FIFA anticipated difficulties the law, emphasising that when in doubt the linesman should not penalise the attacking player. Peter Willis, president of the Referees Association, said: "If any technical aid can be found that might help, FIFA will look at it. We'll have linesmen running around with spacesuits on one day, I'm sure. But they'll still get it wrong at times. There will always be human error."

Perhaps FIFA should be prevailed upon to stress the point again - before the start of this year's World Cup.