Understanding Depth of Field

During this Portraiture project Depth of Field is a key issue, my main target is to understand Depth of Field properly; for example learning the correct aperture, shutter speed and focusing distance. Whilst reading more into the subject I realized that depth of field does not suddenly change from sharp focus to blurry but its a gradual transition. Like the photograph below of Muhammad Ali (1966) which we looked at in one of our first lectures on Depth of Field, you can see his large fist in the main focus of the photograph with his arm becoming less in focus, then his face is more blurred than the rest. Depth of field is important to this photograph as Muhammad Ali was one of of the world's most professional boxers - and his fist is the main focus of the photograph.