How to Determine Focal Length? The Focal Length is the Distance Between the Focal Lens and the Cutting/Engrave Material. When install the Lens, Make Sure the Focusing Lens: Convex Face to Laser, Concave Face to Material. How to Clean Laser Lens and Mirrors? Step 1 Cleaning for Light Contamination (dust, lint particles) Use an air bulb to blow off any loose contaminants from the optic surface before proceeding to the cleaning steps. Step 2 Cleaning for Light Contamination (smudges, fingerprints) Lay the lens tissue on the optic's surface. Using an eyedropper, squeeze a few drops of acetone onto the lens tissue, wetting the complete optic's diameter. Without lifting the lens tissue, drag the lens tissue across the optic just fast enough so that the liquid evaporates behind the tissue. This should leave no streaks. Step 3 Cleaning for Moderate Contamination (spittle, oils) Dampen an unused cotton swab or cotton ball with white distilled vinegar. Using light pressure, wipe the optic's surface with the damp cotton. Wipe excess distilled vinegar with a clean dry cotton swab. Immediately dampen a cotton swab or cotton ball with acetone. Gengly wipe the optic's surface to remove any acetic acid.