Scientists use special lenses to focus electron beams into a tiny point. The outer edges of this converging cone shape form the pencil angle. The shape looks just like the sharpened tip of a wooden pencil.
This specific angle plays a massive role in how the machine works. It directly controls the useful depth of field for the entire microscope. Depth of field means how much of a thick sample looks sharp.
A very narrow pencil angle gives the user a huge depth of field. This keeps tall and short parts of the sample in perfect focus. However, making the angle smaller always comes with a major trade off.
A narrow angle blocks a lot of the electrons from reaching the sample. This restriction makes the final image look much darker and quite noisy. It also reduces the absolute best resolution the microscope can actually achieve.
A wider pencil angle lets more electrons through for a much brighter picture. This wider angle can show much smaller details on a perfectly flat surface. However, the background and foreground of a bumpy sample will look very blurry.
Machine operators must carefully change this angle for every single sample. They use a part called the condenser lens to make this physical change. Students sometimes think a sharper angle always makes a better picture. In reality, the operator must find a careful balance between image brightness and focus.