The aperture diaphragm acts essentially as a control for resolution and contrast in optical microscopy. By varying the size of the diaphragm opening, the illumination cone projected into the objective is changed.

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To see how this affects specimen images, use the slider to open and close the condenser aperture diaphragm. Opening the diaphragm too much results in glare and loss of contrast, while closing it too far results in increased diffraction and loss of resolution. An intermediate position is optimum, which corresponds to an opening size of between 60 and 90 percent. The condenser diaphragm image seen beneath the photomicrograph indicates how the image of the condenser diaphragm appears at the back focal plane of the objective.