For most people new to needing readers, it's highly unlikely that they'll need a high powered lens. Presbyopia like many eye diseases typically starts as a minor vision impediment, and slowly increases as you age. While it's possible for people to need reading glasses that use a strength of 3.5 - 4.0 diopters, glasses like theses are relatively rare, and most people only need low powered readers. Reading glasses are rarely manufactured for strengths greater than 4, and typically range from strengths of 1-3. While reading glasses generally don't need to be as accurate as normal prescription glasses, finding the right size for you is important, as wearing the wrong size can cause headaches, dizziness, or nausea.