Actually, the L series version of the Canon 24-105mm does not have a variable aperture but a fixed maximum aperture of F/4. You should have the same aperture at 24mm and 105mm.
To figure out your problem, first let's make sure you do have the L series lens and not the STM standard one, since the latter would behave the way mentioned in the previous posts.
Yup, no such thing as a free lunch when adding a zoom lens.
The Sigma ART 18-35mm (stunning lens btw) is a fixed f/1.8 - can't sing the praises of this lens highly enough - particularly when coupled with a Metabones Speedbooster (f/1.0!!!)
What is the ballpark cost of this lens if I may ask?
I don't think there's an EF-S to m43 speed booster, so you won't be getting F/1.0 without vignette.
Cool! I didn't know that combo would work.Yeah, I use it with a Blackmagic Pocket with the Pocket Speedbooster - really awesome combo - I have a dumb m4/3 to Nikon adapter too so that one lens gives me a 28-105mm equivalent at a minimum of f/1.8 - very cool.
Looking at the histogram and adjusting the color temp to match, the 105mm is almost exactly 2/10th stop darker. Some of that could be from vignetting, shot wide open, because I had to crop in the 24mm (so the darker edges would've been removed).
I'm not sure why it's darker. It might be one of the quirks of the lens.
The 70-200 2.8 V2 is something like 1/3 stop brighter than other lenses. It's kind of strange.
My two Canon lens come preset with a single F stop. Canon would do well to make all of their lens with adjustable F stops.