The helicoid style eyepieces twist up and down for diopter adjustment and operate independent of the eyecups. This allows them to be left at proper position for your vision. Additionally, telescoping eye cups at the front of the oculars provide 18mm of eye relief, enabling users with or without eye glasses to experience the maximum field of view. How it works: Slit Lamp users wearing eye glasses push the cups in while users without eye glasses extend the cups outward. Both maximize the optical system to view the full image.