The Open Championship is essentially 'open' to any golfer with a handicap index of 0.4 or better, and casting the net out wide means that the field varies greatly each year.
The world's top 50 get in automatically as well as Major winners of the past five years, previous Open winners, qualifiers for the PGA Tour's Tour Championship, top-10 finishers last year and various other tournament winners and high finishers.
