My girlfriend just got her first DSLR and was struggling with changing the aperture. While she understood the concepts, she wasn't familiar with working with a DSLR alone and since I wasn't physically present, it took me a while to realise over the phone she'd been changing the aperture by changing the focal length of the lens. (The D5100 has a pretty crappy and unintuitive control combination for setting aperture.) Had I been able to point her to a simulation like this it would have been clear to her that she needed a distinct method of setting aperture.