In theory, the entire lens would live up to the qualifications set by the client. After all, if a customer orders a 150mm lens, the whole 150mm lens has to be within specification… right? Bad. Indeed, in the real world, nothing can be made exactly as specified. While on paper the whole lens works, in reality the edges of the lens are not to specification. To account for this error, the net aperture is the area of the lens within specification. Usually it is written as a percentage of the goal (such as >95%) or as a quantity such as 145mm.
Surface quality quantifies the irregularities of the lens surface. We don’t want the lens surface to look like the surface of the moon with various dots, ridges and craters. To quantify lens softness, scratches measure the size of the ridges on a lens. The thicker the cut, the higher the amount of scratches. Similarly, digging measures the size of spots or bubbles in the lens. The larger the crater or bubble, the greater the amount of excavation. Normally the scratch is larger than the dig.
Surface flatness measures how close the lens surface is to its specifications. The ideal measurement would be 0, which means that the fabricated lens is exactly within the specifications of the lens on paper. As with a clear opening however, nothing is perfect and therefore the product will deviate from the plan. Surface flatness is measured at a certain wavelength in waves (λ) with values closer to 0 being best and larger values having larger deviations. For example, λ/4 at 400 nm would be better than λ/2 at 400 nm.