<P>Normally NA=0.6 for DVD read-only, focal length is 3.36mm or 3.05mm, hence the clear aperture is different accordingly. The focal length selection depends on requirement for coupling efficiency of Optical Pickup Unit (Head), focus error signal, spot size on photo diode IC, etc.</P>
<P>For R/RW application, NA can be 0.65 or other variation.</P>