Focus refers to the way a camera lens gathers light rays and converges them onto the camera backing sensor.

Portions of an image that are more or less perfectly converged on the sensor are said to be “in focus”, while blurred scene elements are considered to be “out of focus.”

When working with zoom lenses, focus should be adjusted with the lens set to its longest focal length, in other words, zoomed in as tightly as possible. Focus doesn’t change with focal length, so if focus is set when fully zoomed in, the subject will remain in focus at any other zoom setting.