Mounted Large 40 mm Diameter N-BK7 Fast Focusing Singlet, Thick Profile Spherical Lens

a high-quality surplus “Best Form” spherical singlet lens in excellent condition. This is a classic N-BK7 glass positive meniscus/biconvex lens with an asymmetric “best form” design: one side highly curved (nearly hemispherical) and the other lightly curved for optimized on-axis performance and reduced spherical aberration compared to standard plano-convex or equiconvex lenses.

Key feature: The lens is professionally mounted in a sturdy machined aluminum cylindrical housing/cell with a retaining ring, making it ready for immediate use or integration into optical setups — no need for custom mounting!

Key Specifications (approximate where noted, based on careful measurements):

• Clear Aperture/Diameter: Exactly 40 mm (full usable aperture)

• Material: N-BK7 optical glass

• Surfaces: Both spherical

• Central thickness: Approximately 20 mm (very thick profile for stability and mechanical robustness)

• Edge thickness: Roughly 3 mm estimated

• Housing: Machined aluminum barrel (matte silver finish, ~45 mm outer diameter, ~35 mm overall length, with black retaining ring)

• Uncoated (no anti-reflection coating, typical for surplus optics)

Focusing Performance (measured with sunlight for reference):

• Highly curved side facing sun: Produces a bright, intense focus spot approximately 7 mm from the lightly curved vertex.

• Lightly curved side facing sun: Focus spot approximately 15 mm from the highly curved vertex.

• Note: This is a very fast lens (low f/# ~0.8-1.0 at full aperture),

• Best performance with highly curved side toward collimated light.

Ideal Uses:

• Solar concentration experiments (excellent for focusing sunlight — creates a hot, bright spot quickly!)

• High-NA optics projects, illumination systems, or condensers

• Hobby/laser setups (beam focusing/expansion, with aperture stopping for sharper results)

• Educational demos of lens aberrations, caustics, or “best form” optimization

• DIY telescopes, magnifiers, or art/photography effects

• Easy drop-in mounting thanks to the aluminum cell

This lens has no visible scratches, chips, or damage — clean and ready to use.