Deep Cutting lens For CO2 laser (wavelength = 10 micron = 0.01 mm), this becomes: DOF (in mm) =0.027 x ( f / D )2 With a f = 100 mm (4 inch) focal length lens and a beam diameter of 6 mm we get (assume M2=1) spot size = .013 x 100/6 = 0.2 mm = 200 micron and DOF = 0.027 x (100/6)2 = 7.5 mm