When is the actual aperture *larger* than the apparent aperture - uwa lenses?
I was just covering my butt! I suspect there are some unusual designs of lenses where this is true, but even looking at my 17TS-E, far and away the most extreme lens I own, it honours the apparent aperture "rule".
The f stop value of a lens is defined by the focal length of the lens divided by the diameter of the objective lens (front element). so a 300mm lens with a 100mm front element has an f stop value of f2.8. That's how it works.
Basically it is, for simple lens designs like telephotos, but take my 17TS-E, it should have a 68mm front element, I can't measure it to be any more than 64-65mm.
Ever noticed that wide aperture primes tend to under expose by a 1/3 stop when shot wide open? Often it's the difference between the F stop value and T stop value.
Not with TTL metering systems, the underexposure is due to uncollimated light hitting the sensor at angles it can't fully record, which is why it is worse the further off axis, vignetting. This wasn't as big a problem with film as it will expose to light from any direction, but there is so much stuff in front of the sensor that effectively blocks light that isn't nicely aligned; the metering is carried out in the pentaprism and is not affected by this phenomena. The entire point of TTL metering is to take into account all light loss due to lens extension, transmission values etc, the metering works out the exposure based on the light it is receiving, not the aperture value set and ignoring the T-stop or effective f-stop.
I have based my definition of aperture on Wikipedia's Aperture article. http://en.wikipedia.org/wiki/Aperture#Maximum_and_minimum_apertures
It states that "the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane"
I know that Wikipedia articles are community-driven so accuracy of the information might not be 100%, so by all means go ahead and modify it since you know that the information as represented is incorrect/incomplete/inaccurate.
On the occasions I have edited Wikipedia it has been unedited by the people who got it wrong in the first place, so I don't bother.
But if you read this link: http://en.wikipedia.org/wiki/Aperture
You will get to this bit
"In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself. For example, in a telescope the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). One then speaks of a telescope as having, for example, a 100 centimeter aperture. Note that the aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system."
Hope it helps.