Glossary of Light Microscopy Terms
Numerical Aperture (NA)
The numerical aperture is a dimensionless value that characterises the resolution of a lens. It is defined by the following equation:
NA = n sinθ
Where n is the refractive index of the immersion medium and θ is the half-angle of acceptance of the lens.
The higher the NA the higher the resolution of the lens. The NA cannot exceed the refractive index of the immersion medium, so lenses for use in standard immersion oils (n ≈ 1.51) tend to have a maximum NA of 1.4 to 1.45