I am finally starting to get some time shooting with my Canon 7D and my EF 70-300mm f/4-5.6 L IS USM. I really love the lens but recently discovered that the focus lock only works at 70mm. I had started to play with a bit more BIF shooting and wanted to lock the lens at 300 mm but no go! I tried other focal lengths also but the lens only locks at 70mm. It locks with a positive snap at 70mm and does not need to be forced and I use it a lot when traveling with it in my LowePro so I started wondering if the lock might be only for travel. Is this normal?