Which of the following best defines 'numerical aperture'?

Numerical aperture is a crucial concept in microscopy that pertains to the ability of a lens to capture light and resolve fine details in the specimen being observed. It is defined as the sine of the half-angle of the maximum cone of light that can enter or exit the lens, multiplied by the refractive index of the medium in which the lens works. A higher numerical aperture indicates that the lens can gather more light, leading to improved resolution and clarity in images.

The other options describe different attributes of lenses. While magnification relates to how much larger an image appears, and focusing refers to the lens's ability to produce a clear image at a specific distance, they do not directly relate to how much light the lens can capture. Lens distortion pertains to irregularities in image quality due to the physical properties of the lens itself, which also does not connect with the definition of numerical aperture. Thus, understanding numerical aperture is fundamental for evaluating the performance of optical systems in biological microscopy.