The assertion by a well known photographer is that the Nikon 400mm f/2.8 set to an aperture of f/5.6 will let in 4 times the light compared to a Nikon 500mm f/5.6 lens set to f/5.6, keeping the shutter speeds identical.
The ultimate conclusion from this was that the focus acquisition time was much faster for the 400mm lens at f/5.6 than the 500mm set at f/5.6 due to more light.
While there may be different transmission factors for the glass, especially given one lens is the Nikon 500mm PF, I don’t believe the difference would be 4 times. He seems to be asserting that the faster lens is faster at any aperture when compared to a slower lens.
I would expect that if both lenses are set to f/5.6 and all other settings were the same, there should be less than a 1/2 stop difference in light transmission.
Where does the large difference in the statement of the photographer come from?