The condenser's function is to focus light on the object. For maximum visibility of details, the field also needs to be uniformly lit.
The condenser is positioned in such a way that its front lens almost touches the slide. To examine objects at a low power, the front lens can be taken off some condensers. By doing so, the lighting becomes smoother and more evenly distributed.
Every condenser has an iris diaphragm, which sorts the rays of light that pass through the middle of the collecting lens. By shutting down the iris, scattered light is minimized and the contrast and field of depth are boosted. However, this maneuver diminishes luminosity and resolution.