In optics, an aperture is a hole or an opening through which light is admitted. The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane. If the admitted rays also pass through a lens, highly collimated rays (narrow aperture) will result in sharpness at the image plane, while uncollimated rays (wide aperture) will result in sharpness for rays with the right focal length only. This means that a wide aperture results in an image that is sharp around what the lens is focusing on and blurred otherwise. Obviously, the aperture also determines how many of the incoming rays that are actually admitted and thus how much light that reaches the image plane (the narrower the aperture, the darker the image).
In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.The exact definition of the term varies slightly between different areas of optics. Numerical aperture is commonly used in microscopy to describe the acceptance cone of an objective (and hence its light-gathering ability and resolution), and in fiber optics, in which it describes the cone of light accepted into the fiber or exiting it.