In reality the electrons have helical trajectory in the lenses as shown in Figure 2702a. The electromagnetic lenses consist of windings of wire through which the electric current is applied. These configurations create a strong magnetic field where the incident electron beam must pass. The electrons follow a helical trajectory which converges at a fine focal point after they emerge from the lens. Here, the DC(direct current)-powered magnets behave similar to converging glass lenses in optical microscopes.
Figure 2702a. Schematic illustration of electrons passing through electromagnetic lenses.
When the current of condenser lens is changed, e.g. with BRIGHTNESS knob to change the second condenser lens (CL3) current in commercial TEMs, the illuminating spot size and intensity on the viewing screen will be changed as shown in Figure 2702b. Therefore, the focal point of the condenser lens can be above, on, or below the viewing screen. Note that the intensity becomes weaker when the illuminating spot size becomes bigger.
Figure 2702b. Effect of change of condenser current on the illuminating spot size.