CoreMotion: detect phone moving forward and backward

I would like to implement zoom feature for my app by using Core Motion. The scenario is when user hold iPhone 90 degree to the ground, user move iPhone far way from their eyes, would have zoom out feature. And when user move iPhone closer to their eyes, would have zoom in feature.

I understand that I should use userAcceleration values to calculate the distance that iPhone move. But actually I don't really understand the difference between acceleration and userAcceleration getting from CoreMotion.

Did anyone implement this zoom feature before?

Thanks