So once again a pro has posted some instruction on pre-shot routine.  Lee Brett gives a nice example of field of vision while talking about how you should always approach the table and not start at the table.  For each shot you should always start from a couple steps back from the table so you can get a better picture of the entire table.  You never know when you may see a ball near the closest rail you are at that is a better shot to start with than the one you are currently looking at.

Lee gives an example of putting your hand one inch away from your face, then put it 6-8 inches away.  You will see that it is blurry at one inch but clear and complete at 6-8 inches. This is kind of the same principle because as you maybe able to see your hand the entire picture is truly blurry until you are able to take a step back to see the entire thing.

So before you even think about getting down to take the shot, first take a step or two back from the table for every shot to make sure you are making the one you should.  It could be the difference between seeing the run out and just seeing the next shot.