The electrons from the gun come out in almost a spray pattern, so we may have a flow of electrons, but this could hardly be called a beam. As in a light microscope, we need lenses to control the flow of electrons, however, the glass lenses of a light microscope will not work. Instead, an electron microscope uses electromagnetic lenses.

An electromagnetic lens is a relatively simple device. By applying current to wire coiled around an iron cylindrical core, a magentic field is created which acts as a lens.

The advantage an electromagnetic lens in an electron microscope has over its glass counterparts in a light microscope is that by varying the current through the wires, the lens can have a variable focal length.

   
       We now can arrange the electron gun and lenses in a column mounted on a sample chamber.
The condenser lens controls the size of the beam, or the amount of electrons travelling down the column. Increasing the size of the beam achieves a better signal to noise ratio, but because the beam diameter is larger, gives a lower resolution. Depending on the magnification, a compromise between signal to noise and resolution achieves the best image quality.

The objective lens focuses the beam into a spot on the sample. This is necessary to have an image in proper focus.

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