In an effort to avoid the problem I have put a 'tag' of electrical tape sticking out the side of the cap. The idea is that you will see it when you look thru the viewfinder. Only proviso is the cap has to be put back on so it lines up with the viewfider. I do find however, that it helps me notice the cap even before sighting up and especially as I adjust the f stop

Murray
Brisbane, Oz.