Diagnosis
Dust and fingerprints degrade image quality. The lens has a special coating that's easily damaged.
How to Fix It
Use a rocket blower (like for camera lenses) first to remove loose dust. For smudges, use a microfiber cloth designed for glasses or screens. Gently wipe in a circular motion from the center outward. No liquid needed for most marks.
If That Didn't Work
For stubborn spots, lightly moisten a corner of the microfiber cloth with distilled water, not tap water. Wipe gently, then immediately dry with the dry part of the cloth.
Heads Up
Never use window cleaner, alcohol, compressed air (can blow dust under the lens housing), or rough paper towels. The coating is softer than glass and will micro-scratch.