I was thinking this: the 45/1.8 is an extremely small lens, with a 37mm filter thread. Then you have the small 40-150 with a 58mm filter thread. So you should have plenty of space in the front element to get the light for a 1.8 on the wide end of the lens. Given that no such beast exists, what does it happen exactly? Why/where is all that light lost? Or, in other words, why zooms require so much more light?