Shorter Focal Length - Smaller Spot - Higher Energy Density - Deeper Marking? This article addresses a common technical misconception in laser processing: the belief that a shorter focal length lens is always superior for deep engraving due to its smaller focused spot and higher energy density. Through rigorous mathematical derivation and data analysis, the author demonstrates that Depth of Focus (DOF), rather than initial spot size, is the decisive factor for successful deep engraving.