The Ability to Resolve Spatial Light Variation Is an Integral Part of any Imaging System. For the Purpose of Differentiate Between Spatial Variation on a Plane Perpendicular to the Optical Axis and Variation Along the Optical Axis Inside a Camera Behind the Main Lens. the Former Quantity, Transverse Variation, Is What All 2D Sensors Measure. Light Variation Along the Optical Axis Can Be Described As the Depth of Field of an Imaging System.