In a short-sighted eye the lens is too powerful, or the eyeball too long, so light from a distant object focuses in front of the retina. In a long-sighted eye it is the opposite: light focuses behind the retina. A concave (diverging) lens spreads light out first to correct short sight, and a convex (converging) lens brings it together sooner to correct long sight. Choose the lens that lands the focus on the retina.