Equally important to the lenses are the apertures. These are circular holes in thin strips of heavy metal, placed at well-chosen points in the column of lenses. Some are fixed in size and position and play important roles in limiting x-ray generation and improving the vacuum performance. They also prevent electrons from passing through the outermost parts of the magnetic lenses which, due to large lens aberrations, focus the electron beams extremely poorly. Others can be freely switched among several different sizes and have their positions adjusted. These "variable apertures" are used to determine the beam current reaching the sample and also to improve the ability to focus the beam. Variable apertures after the sample position further allow the user to select the range of spatial positions or electron scattering angles to be used in the formation of an image or a diffraction pattern. Skillfully used, these apertures allow remarkably precise and detailed study of the defects in crystals.