I used the Panasonic G9 camera to find the Panasonic Leica DG Vario-Elmar 100-400mm f/4-6.3 maximum aperture value for each focal length. See the video below as I turn the 100-400mm lens with it set “wide open” and get the f-stop number.
I made this video and charts so people can compare how much light they can get in the Panasonic 100-400mm lens at each focal distance. This will also let you compare your existing lens like the LUMIX G VARIO Lens, 100-300mm, F4.0-5.6 II, Panasonic 45-200mm, F4.0-5.6 II, 45-175mm, F4.0-5.6 ASPH, 14-140mm, F3.5-5.6, or 35-100mm / F4.0-5.6.
The light gathering abilities of this lens is OK for having such a long telephoto reach (800mm Full Frame Equivalent). But, nothing like the Olympus M.Zuiko Digital ED 40-150mm f/2.8 PRO. It will give you an F2.8 aperture whereas the Panasonic is only a F4.6 at 150mm focal distance. That is more the double the amount of light the Panasonic can take in. But, then again, the Panasonic can zoom on out to 400mm. You will have to weigh which will be a benefit for your type of photography. Do you need the distance of the low light shooting?
The Max Aperture automatically goes up when you reach these focal distances. I tested this over and over and got the same results. At no time did the lens ever register at F6.0 or F6.2 during my many sample turns neither up nor going back down.