Figure 1. A modern classroom microscope with mechanical stage and binocular head.

Figure 2. Defining numerical aperture (NA).

Figure 3. Images of the diatom Pleurosigma angulatum, to illustrate the function of the condenser aperture iris diaphragm. The objective used was a ×40, 0.65 NA achromat. The field size is 60 m × 80 m. In (b) aperture iris set to 7/8 of the objective NA as seen in the objective back focal plane. The image has optimal compromise between MRD (the ability to resolve the line pattern) and contrast. In (a) aperture iris opened too far. Flare has resulted in loss of image contrast, although the pattern can still be resolved. In (c) the aperture iris closed too far. Contrast and depth of field have been increased, but the effective objective NA has been reduced, the MRD adversely affected and the pattern is no longer resolved.