In order to understand the focusing action of a TEM objective lens, a simple imaging system (figure 1) is best considered. This system consists of an objective lens and a single projector. In operation, the projector is adjusted as required within the total imaging system to achieve a magnification, M2, on the screen. This results in a focal length of F1, with the lens seeking to find an image in the position M1. If the objective lens does not place the image at M1 the result on the screen is an out of focus condition. The objective lens may have produced an image short of M1, overfocus, or beyond M1, underfocus.