<P> In microscopy, NA is important because it indicates the resolving power of a lens . The size of the finest detail that can be resolved is proportional to λ / 2NA, where λ is the wavelength of the light . A lens with a larger numerical aperture will be able to visualize finer details than a lens with a smaller numerical aperture . Assuming quality (diffraction - limited) optics, lenses with larger numerical apertures collect more light and will generally provide a brighter image, but will provide shallower depth of field . </P> <P> Numerical aperture is used to define the "pit size" in optical disc formats . </P> <P> Increasing the magnification and the numerical aperture of the objective reduces the working distance, i.e. the distance between front lens and specimen . </P> <P> Numerical aperture is not typically used in photography . Instead, the angular aperture of a lens (or an imaging mirror) is expressed by the f - number, written f / or N, which is defined as the ratio of the focal length f to the diameter of the entrance pupil D: </P>

Define numerical aperture of a step index fibre