Magnification is the factor by which the angular diameter of an object is apparently increased when viewed through a telescope, microscope, or other optical instrument. It can be calculated by dividing the focal length of the telescope by the focal length of the eyepiece. The best magnification to use depends on the type of observation and on the seeing conditions. High magnification may be necessary, for example, for separating close double stars or for resolving fine detail on a planet's surface, but it can also be a disadvantage. It results a smaller field of view, a dimmer image with less contrast, and the emphasis of any shortcomings in an instrument or atmospheric disturbances. As a rough guide for the amateur, the practical upper limit to a telescope's magnification is twice the instrument's aperture in millimeters. A lower limit to magnification is set by the size of the exit pupil. When this exceeds the size of the pupil of the eye, light is wasted and the image appears no brighter than if the magnification were increased.