Just starting out with lacrosse pictures on my new Canon EOS 60D and zoom lens EF 70-300mm f/4.0-5.6 IS USM. Why can't I set aperture at 4.0 and it stay there? Why does the aperture get smaller as I zoom in? I just discovered that this happens. I just thought I couldn't dial to 4.0 and couldn't figure out why wouldn't go any larger. Doing all in Manual mode. I hope this isn't as stupid as it sounds, but I think when I learn the answer I will have a lightbulb moment and really grasp this. Thank you for anyone's input.