When looking at the operation of the deflection circuit , we see that it constantly adjusts the strength of the deflection yoke’s influence on the electron beam as the length of its path to the faceplate, and the amount of time the beam spends under the influence of the deflection yoke and the anode, varies.
The focusing circuitry, likewise, has to take these variables into account to ensure that the beam is correctly focused, no matter where on the screen it lands. To achieve this, additional electrodes are added to the basic focusing system illustrated in beam-focusing. Signals taken from the deflection amplifiers are superimposed on the voltage fed to these electrodes, altering the strength of their electromagnetic field in step with the adjustments made to the strength of the deflection yoke’s electro-magnetic field. This process of constantly changing the CRT’s focusing characteristics is known as dynamic focusing.