The objective lens forms an inverted initial image, which is subsequently magnified.

In the back focal plane of the objective lens a diffraction pattern is formed. The objective aperture can be inserted here.

The effect of inserting the aperture is shown on the next page.

The objective lens would not usually provide a magnification of more than ×50 and a TEM is routinely used to view regions of the specimen which are only a mm or so across. What does this imply about the diameter of a typical aperture if it were placed at the back focal plane as shown in this diagram?