There are two categories of lenses, both distinguished by a different numerical aperture: SLW (wide)and SLH (high). Each NA type is appropriate for different applications and exhibits different alignment sensitivities. C18 is optimized for Single Mode Fiber application to have better coupling efficiency. So, when you consider the usage of SMF, C18 have better Insertion Loss performance than W18.
While the numerical aperture is a maximum at the center of the lens aperture, the actual NA is a function of the ray parameters (height and angle) for each ray that strikes the lens’ surface. Click to the Technical Guide of this guide for an illustration of how the NA depends on these parameters.
(Click on Image to view in full screen)
(Common Characteristics Chart) (Optical Parameter Chart)