Here is a nice article about from ZEISS about the irradiance and apertures of camera lenses.
Lens aperture is – in addition to film or image sensor sensitivity – responsible for the degree of irradiance striking the image plane.
In a detailed technical article ZEISS expert Dr. Vladan Blahnik shows why lens aperture depends upon object distance and explains the underlying mechanisms for several optical designs of ZEISS lenses.
With many concrete illustrations the reader also learns why a lens should be panned around the entrance pupil, why anti-reflection coatings are so important for image quality, how lens focusing with floating elements is realized or why optical designs strongly depend upon the available space as given by the camera layout.
Get the full ZEISS release HERE.
Pic is from ZEISS.