While many photographers, both professional and enthusiasts, know the rule of thumb of focusing on someone’s eye for portraits in order to achieve “critical focus”, not everyone understands the difference between critical focus and depth of field, two very related but very different terms. Critical focus is the part of the photo which is optically in focus. It is based on a simple formula which takes only two variables in consideration: the focal length of the lens, and the distance between the back of the lens and the film/sensor. The result is the distance at which critical focus is achieved, and anything on that plane is called critically in focus, while something even 0.0001mm in front or behind that distance might still be extremely sharp, but we wouldn’t be able to call it critically in focus.
In contrast, the depth of field adds a few other important factors: aperture, which affects how much of the image is in focus, and the circle of confusion, which defines what we consider as adequately in focus.
So the depth of field applies to a range of distances, for example with a Canon T5i with a 18mm lens focused at 1m at f/8, the area which is acceptably in focus is everything between 0.696m and 1.774m. In contrast, only something which is exactly at 1m distance would be considered in critical focus.