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Adjusting the condenser diaphragm is crucial for optimizing contrast after performing Koehler illumination. Koehler illumination is a technique that ensures even lighting across the field of view and enhances the resolution and contrast of the specimen being observed.

The condenser diaphragm is responsible for controlling the amount of light that passes through the specimen. By adjusting the diaphragm, the user can increase or decrease the light intensity and alter the light's aperture, which directly affects the contrast of the image. A properly adjusted diaphragm allows for more precise control over how much light is allowed to illuminate the specimen, thus improving the visibility of fine details and structures.

In contrast, the primary lens, stage position, and light source play different roles in microscopy but do not directly adjust the contrast in the same manner as the condenser diaphragm. The primary lens primarily focuses the image, the stage position centers the specimen, and the light source provides illumination but does not inherently provide contrast adjustments. Therefore, the condenser diaphragm is the appropriate component to adjust for optimal contrast in the context of Koehler illumination.