You know it might be time to change the lens, if…

Don’t run – we come in peace, a photo by stst31415 on Flickr.

…you have the impression you reached a point where you do not learn new things with the lens you use at the moment.

Maybe there is also a time for changing the point of view. It does not really matter. I had the 50mm/1.8 on my camera for a longer time. I need a change.