I think that what you are seeing is a side effect of internal focussing. You will probably find that at infinity focus each lens is approximately 300mm at the long end. But as you focus closer, without moving the zoom control, you will see that the focal length reduces. This is something to be aware of. For example, the Nikon 70-180mm zoom macro lens is more like a 100mm lens when focussing in the macro range with the zoom set to '180mm'.
Leif
|