Agree, but why would you be concerned about the absolute magnitude of the opening of the diaphragm ?
The f number describes the speed, and the focal length describes the usage. As a photographer, I am not sure why one would be concerned about the absolute size of the opening at all.
When you hit the shutter button there are 3 things that happen, the diaphragm/shutter closes and opens up, the sensor starts recording the image and in case of an SLR the mirror flips out of the way to allow light to reach the sensor.
The amount of light captured depends on 3 things, which most photographers jargonize as the triangle of exposure. The shutter speed shows how long the shutter remains open (1/60 is 1/60th of a second or 0.1 sec ).
The f stop gives us the diameter of the opening (f/2.8 means that at 28mm focal length the diameter of the opening will be 10mm), this means that the higher the fstop or lower the so called fstop number, more light comes in if the shutter remains open for the same amount of time. This means that you can take the same pic with a much faster shutter speed and hence lenses with lower fstop numbers f/1.8 or even f/1.2 are called fast lenses. BTW the higher the aperture opening the bigger the lens needs to be and hence more glass, that is why photographers are so hung up on the term glass, viz "this is a better glass" "look at all that glass"
The third is the ISO, which basically means how much of the image the image is enhanced or guessed by the camera. A high ISO means that you can get the click with faster shutter speed but there will be less detail since most of the image is not captured, it's guessed by the camera. This is the same thing that happens when you increase the brightness of your image on a computer
A high gyan explanation of the above: *www.dpchallenge.com/tutorial.php?TUTORIAL_ID=45
An amazing simulator as to what settings do what : *www.canonoutsideofauto.ca/
