All lenses are not created equal
Q: DO TWO LENSES—ONE PRIME AND THE OTHER A ZOOM—LET IN THE SAME VOLUME OF LIGHT GIVEN THE SAME APERTURE? WILL AN 85MM PRIME AT F4 GIVE THE SAME EXPOSURE AS A 24–105MM ZOOM AT F4? DOES THE LENS CHANGE EXPOSURE?
A: An 85mm at, say, f4 should let in the same amount of light that a 24–105mm lens at f4 brings in. “Should” being the key word. In the real world, though, you may find this is not the case.
Manufacturers are often allowed a +/– 1/3 stop variation in manufacturing tolerances. That means that one lens at 2.8 could be +/– 1/3 of a stop from another lens at 2.8. You could own two lenses that are possibly 2/3 of a stop off from each other.
You may come across an old, used lens one day that has “+1/2” or “–1” scratched onto the barrel of it. In film days you would test and test and test all of your gear and find out where lenses fell compared to your light meter. Then you would know that with one lens you needed to add 1/2 a stop, and with the other lens you would need to decrease one stop.
My 24–70 2.8 and my 70–200 2.8 are at least 1/2 stop different from each other.
In a perfect world, 2.8 would be 2.8 no matter what lens. In the real world, there can be a fairly wide gap between the same aperture settings on different lenses. The best thing to do is test all of your gear.