i have really unusual problem with my new canon 5d mk3.
i was shooting video, using 24-105mm f4 canon lens, no filters, manual settings both lens and camera, at an aperture f9 shutter 60 at 24mm. when i zoomed to 105mm i noticed that overall amount of light changed as if it was stopped down one and a half stops while the setting remained at f9?! so i had to adjust aperture to get the proper exposure. i checked the recorded material and the change in light was also recorded, thus eliminating monitor preview. i went through all settings and double checked that everything is set to manual. i went to a friend and tried my lens on his 5d mk2 and everything was in order. i tried his lens canon 24-105mm on my camera and it did the same mistake. then we tried canon 70-200 f2.8 and it worked properly. i also tried installing new firmware and everything was the same. i tried different combinations of apertures and shutter speeds and it was still there. here is a link to youtube video that shows the malfunction.https://www.youtube.com/watch?v=xvC1A2BsGms