In my case is different. I tend to focus and learn better with cannabis, but that might be that it relaxes me to some degree that I can take existence better and concentrate stimulating my mind.

In fact I believe weed changed me for the better. May sound cliche but now I'm deeply interested in philosophy and technical concepts and aptitudes (like programming), and actually my whole attitude towards life got really bright and mature, with myself and with others.

I'm not saying weed alone made that, but definitely helped me. As I said, at least for me concentration, in this case facilitated by weed, is life changing.