In the resolution formula, what does "NA" stand for?

In the context of microscopy, "NA" stands for Numerical Aperture. This term defines the range of angles over which the system can accept or emit light. The numerical aperture is crucial because it influences the resolving power of an optical system. A higher NA indicates a greater capability of the lens to gather light and resolve fine specimen detail at a fixed object distance. This directly affects the clarity and detail of the images produced by the microscope.

Understanding numerical aperture is essential for anyone studying microscopy, as it quantifies the light-gathering ability of the microscope objectives and is critical for optimizing imaging techniques.