"Honestly I only half understood it."
No problem let's try another approach. Do you have a binocular at home that you use? It is just a lens. Just like your camera lens. It has a constant f-ratio of, let's say f4. However they do not have a means of reducing the amount of light coming into the binoculars and the amount that comes out for your eye to see. Now if your camera lens did not have that extra system, the aperture blades, it would work the same way.
"But it raised another question to me. The lens says F4 constant aperture and I paid a lot for that ... I am expecting it to stay constant at f4 :-)"
There are two different points here. Did you mention what lens you are talking about? It really doesn't matter but let's assume it was a ef 24-105mm f4L (constant aperture) zoom. When you zoom any lens, say, from 24mm to 105mm you change the focal length and therefore you change the f-ratio. Some lenses do not have special internal systems to cope with this so their f-ratio changes. An example might be the ef-s 55-250mm f4-5.6. However, a lens like the ef 24-105mm f4L has internal lens groups to correct for this and its f-ratio remains constant.
Now where you seem to jump the track is when we have to add exposure to this equation. All the pictures you take can not be shot at the constant f4. So, how do we solve this dilemma? We add an aperture system that has the ability to reduce the amount of light that hits the sensor. These are called f-stop numbers and although they sound like the same number as your constant aperture f4 lens they are not. Bottom line here is your constant f4 lens shot the scene at f4 just like Canon claimed but the camera told the lens aperture system to reduce the amount of light to f9 because that is what made a perfect exposure. If you removed the aperture system from your constant f4 lens, it would shoot all shots at f4 and some would be over exposed.
Consider this, as we wrap this up and I hope I made it more clear for you, all prime lenses are constant aperture lenses. Zoom lenses can be either constant or variable aperture but both still have the aperture system to adjust light coming in and going out. Two, there is nothing inherently better in a constant aperture zoom lens over a variable aperture zoom lens. And, the down side is cost and weight because they need that extra lens grouping inside to keep the f-ratio constant. Plus they generally are faster lenses and that costs more.
EB
EOS 1DX and 1D Mk IV and less lenses then before!
