1. Material

• Core: Precision-crafted from high-grade optical glass (e.g., BK7, Fused Silica) or specialized optical plastics.

• Coating: Multi-layer Anti-Reflection (AR) coating deposited onto both concave surfaces. Common coating types target specific wavelength ranges (e.g., Visible: 400-700nm, NIR).

2. Key Properties

• Shape: Features two inward-curving (concave) surfaces, causing light rays to diverge.

• Focal Length: Has a negative focal length (e.g., -25mm, -50mm, -100mm).

• Optical Power: Acts as a diverging lens.

• AR Coating Effect: Significantly reduces surface reflections (often to <0.5% per surface), increases light transmission (typically >98% per surface in target band), minimizes ghosting/flare, and improves contrast.

3. Primary Function

• To diverge incident light rays. Parallel rays entering the lens spread out as if originating from a virtual focal point in front of the lens.

• To correct optical aberrations, particularly in compound lens systems (e.g., compensating for convergence/spherical aberration caused by positive lenses).

• To increase the effective focal length of optical systems when placed before a primary lens.

• To reduce overall system length in some designs.

4. Critical Applications

• Cameras: Used in telephoto lens groups to extend focal length; in zoom mechanisms; for aberration correction; and as field flatteners. AR coating is vital for image clarity and light efficiency.

• Projectors: Essential for beam expansion, aberration correction (especially in projection lenses), and focal length adjustment within illumination and projection paths. AR coating maximizes brightness and contrast.

• Telescopes: Employed in eyepieces (e.g., Huygens, Ramsden designs) for magnification and field correction; in Barlow lenses to increase effective focal length/magnification; and for collimation adjustments. AR coating enhances transmission crucial for faint celestial objects.

• Laser Systems: For beam expansion and optical path control.