Numerical Optics Apertures

http://www.google.ca/search?hl=en&q=%22numerical+aperture%22+%2Blens&btnG=Google+Search

For a practical viewpoint, try a book on photography?

In fact its modern camera terms. the solid state sensor camera is popular and it has a smaller aperture diameter and focus distance than tranditional machine type(film type). To get a equavalent flux and it can be strenghened by elecronic circuit. so it have terms of digital aperture, digital zoom. as we all know aperture (index) = 1/(f/D).we have terms : equal to. yes you can refer most of ordinery photo book. most of digital camera can be aperture of 2.8, some can be up to 1.2.

I would recommend you at least start with a book that precedes digital photography, as many of the newer books read as if they were designed to be confusing.

Fyz