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” ...