For many new photo hobbyest, this area is the most confussing. They look at these numbers, and don't see a conection. There is a conection, and it can be seen if you understand a little about fractions, and how they affect part of the overall exposure.

First off, I'll explain what each is...
  1. f/stop. This is a Fraction that represents the real iris opening compared to the focal length. Example: f/2 on 50mm lens (the size of the sensor has NO baring on this at all). f = the focal length, in this case, 50mm, so lets complete the fraction f/2 for a 50mm lens: = 50/2 so then 50mm/2 or 25mm, equals the iris opening. which is 25mm for a 50mm set at f/2 FOR ANY SENSOR SIZE!!! That NEVER CHANGES. The sensor size DOES NOT CHANGE THIS.. Though, it does change other qualities... but, not this one..
  2. Now, the f/stops were set up to originaly, to be spaced by double or 1/2 of each step.... IE: f/2 to f/2.8 equals a 1/2 step in light gathering power. (it is close enough for exposure, but, the math geeks can show it is not exactly 1/2). f/2.8 to f/2 equals a 2x step in light gathering power.
  3. f/2 for a 50mm lens = 25mm Iris Opening
  4. f/2.8 for a 50mm lens = 17.8mm Iris Opening ( not 1/2 in numbers, but 1/2 in light gathering power)
To add a few more samples:
  1. 100mm f/2 lens: 100mm/2 = a 50mm Iris opening, that = 's the same light gathering of a 50mm f/2 lens, even though the iris opening is 25mm.
  2. 25mm f/2 lens: 25mm/2 = a 12.5mm Iris opening, that ='s the light gathering of a 100mm f/2 lens, even though the Iris is only 12.5mm in size.
So, the relationship with WHOLE f/stops is 1/2 or 2x in steps from each other. Remember this relationship if nothing else.