What Does the Condenser Do in a Microscope?

The condenser optimises the light from the microscope's illumination system so that it reaches the front lens of the objective at the correct angle illuminating it completely. This is essential for achieving the best possible image quality (resolution, fine details).

A key component of the condenser is the aperture, which functions much like the aperture in a camera lens. Closing the aperture increases contrast and depth of field, but too much closure can result in the loss of fine details and introduce unwanted artifacts, such as diffraction rings, colour fringes, and dust specks into the image.

Close-up of a microscope condenser with adjustment screws and lighting

How the Aperture Affects Image Quality

The effect of aperture adjustments can be seen in the following image series of a finely structured diatom:

Series of microscope images of a diatom with a gradually closed condenser aperture, showing changes in resolution, contrast, and depth of field. Dust spots become visible at high depth of field

You can find a higher-resolution version of the image here

In the leftmost image, the aperture is wide open: The finest structural details are visible, but the image shows low contrast and a shallow depth of field. The insert shows a digitally enhanced version with increased contrast. Moving toward the right of the image series, the aperture is gradually closed. In the rightmost image delicate structures have disappeared or are concealed by artifacts. Dust particles on optical surfaces outside the focal plane become more prominent. In extreme cases, even the observer’s eyelashes or floaters (“muscae volitantes”) can appear in the field of view.

In this example, the optimal condenser setting—balancing resolution, depth of field, and contrast—is found in the second image or somewhere between the second and third images.

Practical Tips for Adjusting the Aperture

When scanning a specimen, such as in medical diagnostics, ensuring no important details are overlooked is the top priority. In these cases, a greater depth of field is preferable, even at the expense of fine details—meaning the aperture should be more closed. However, when switching to higher magnifications, it's important to open the aperture again.

Tip: Achieving the Best Illumination with Köhler Illumination

For the best possible illumination, microscopes should be set up using Köhler illumination. Even users with little experience can achieve precise illumination settings with this method. To use Köhler illumination effectively, the microscope must have a height-adjustable and centerable condenser, a condenser aperture, and a field diaphragm to control the illuminated area.