Comment by lusr

No see you're as confused as she was :) Most zoom lenses are variable aperture, e.g. at 18mm they will open up to f/3.5 but at 55mm they will only open up to f/5.6. The simulation doesn't address this, and it can be surprising for a beginner to see the aperture changing as they zoom - they may become fixated on zooming as a way to control aperture without realising they are observing a side-effect.