A transmission electron microscope operates in much the same way as a light microscope, using magnetic lenses instead of glass lenses and electrons instead of photons. A condensing lens focuses a beam of electrons emitted by an electron projector into a small spot 2-3 um in diameter on a sample; after the beam passes through the sample, it is focused by an objective lens to obtain a projection of the sample's magnified image on a special screen or sensor. One of the microscope's key elements is the aperture stop located on the rear focal surface of the objective lens. The aperture stop defines the image contrast and the microscope's resolution. The image contrast in a TEM occurs because some electrons are absorbed by the material as the electron beam passes through the specimen. The thicker a certain area of a specimen or the heavier the atoms, the more electrons become scattered. If the aperture stop effectively intercepts scattered electrons, thick areas and areas with heavy atoms will appear darker on the resulting image. Lower aperture increases contrast, but results in a lower resolution. Elastic electron scattering in crystals results in diffraction contrast.