Reading a review of the new 100-400 II a comparison shot showed that the new lens gave a slightly smaller image than the original when focussed at some distance (I don't have the information but would not have been at infinity).
I compared the image magnification between a 100-400 I and II using two sheets of A4 paper pinned to a tree folded to be 580mm tall (this should just fit an APS-C sensor at 10m). The mk II was indeed slightly smaller than the mk I giving effective focal lengths of 360mm and 370mm respectively. Being rather less than the 400m stated on the tin, I compared moon shots. With a 2xIII extender on both, they were a lot closer. The diameter of the moon measured 7.08mm (assuming that the sensor height was 14.9mm) with the Mk I and 6.98mm on the Mk II. NASA data showed that the moon diameter covered 1837 arc seconds at the time the image was taken. So assuming that the lens effective length is 800mm, the image size should have been 800mm x 1837/3600 and adjusted for radians gave me 7.12mm, making both lenses appear very close to 400mm, with the Mk II being 392mm on that basis. But focus breathing seems to be slightly worse.
Anyone else noticed?