Originally posted by thePiRaTE!! Thanks Sean. I kinda wonder about that... could one theorhetically simply add a stop to an inferior glass and have perform poorly, yet leave one the option? I've noted that the larger apertured lenses aperturers are, in fact, phyiscally larger. Is that a necessary rule to a larger aperture rating?
You are right on the track: Aperture is defined as Focal length of the lens divided with the open diameter. So, the bigger the diameter, the smaller the f-number gets, the faster the lens.
In reality the "open diameter" is only a rough estimate, the entrance pupil is the really defining factor. But that is only to be scientifically correct.
Example: Pentax Macro (old) 4.0/100 gives a diameter of 25 mm (roughly 1 inch)
Pentax macro (new) 2.8/100 gives a diameter of 35,7 mm (roughly 1.4 inches)
Lenses often get better, when you step the aperture down app. 2 steps. (There are only very, very few lenses, which have top-quality fully open). If you mask down the diameter of the front lens (cut a mask out of black board) you have basically the same effect. Your open diameter gets smaller, the f-number gets bigger and the quality is improved. This is, because you cut out those rays of light, that go through the outer peripherie of the lens - and this is the area of the lens, where most aberrations are produced. Also, cutting out the peripheral rays means, that you loose those rays which are bundled under very oblique angels, which als reduces aberrations (especially coma and colour fringes).
regards
Ben