There are many default definitions in the MIL standard. For example the grade that must be required outside the clear aperture (the part of the lens to which the standard applies, also called "effective diameter" or CA) is, in the absence of another definition, 80-50. This is a very basic surface characterization and is easy to achieve. It describes a scratch whose brightness is less than that of a scratch at visibility grade 80 and a dig with a diameter of up to 0.5 mm (50 hundredth = 50/100=0.5). 60-40 is considered "commercial" quality, while for demanding laser applications 20-10 or even 10-5 are used. The scratches on a 10-5 or 20-10 surface can be hard to see, making the visibility standard more subjective. Other standards may work better when precision surfaces are required. Optical coating can change scratch visibility, so for example an element that passes 40-20 before coating can be worse than 60-40 after coating.