Reduce transparency and Lock-screen Background
I have had this question as well for a long time and my answer is — as far as I can tell and have tried and researched in other posts and fora — there is no way to do so. Hopefully that assertion will motivate someone to try and find an answer that proves me wrong. 

As of 2017 the issue you are having is also present in MacOS Sierra — to respond to the claim that it is unique to El Capitan — making the issue still relevant. I have asked this question myself recently on apple.stackexchange but nobody has responded as of yet.