The JEOL JEM 1010 has three apertures that the user can change: apertures for the condenser lens (CL), the objective lens (OL) and the intermediate lens (IL, also often called SAA - selected area apertures). The basic design of the aperture holders is the same in all three cases, and is shown in detail in the line drawing to the right.
An aperture strip contains the actual apertures (holes in a thin metal piece) and is extended into the path of the electrons by flipping the lever shown in the photograph of the IL (SAA) aperture holder below: when the lever points to the right, no aperture is inserted, and when it points to the left, an aperture is in place. The individual apertures are selected by turning the large knurled knob (marked in the line drawing), and can be centered using the knob to the right of the large knob and the smaller knob at the end of the large knurled knob (both marked in the line drawing). As shown in the photograph, the relative size of the different apertures is shown by the size of the white dots on the large knurled knob, and the chosen aperture is the one closest to the black dot near where the lever pivots.