Field Stop Diaphragm Function

When the microscope is properly set up and aligned for Koehler Illumination, the field stop diaphragm – matching the imaged object field diameter – serves to reduce false light within the beam path, avoiding a contrast loss.

Key Learnings:

• The field stop diaphragm is imaged into the specimen plane and other image planes, forming a set of optically conjugated planes.
• The correct diameter of the field stop diaphragm is determined by the objective magnification and the eyepiece field of view (FOV) number.
• For digital images and critical brightfield applications, it is recommended to match the field stop diaphragm diameter to the camera sensor size for optimum false light reduction.

The field stop diaphragm is imaged into the specimen plane and the consecutive image planes; the intermediate image plane followed by the camera sensor/retina plane. We say that:

• the field stop diaphragm plane,
• the specimen plane,
• the intermediate image plane,
• and the sensor /retina plane

are optically conjugated to each other. They form a set of optically conjugated planes. The diameter of the field stop diaphragm is set with that of the imaged object field diameter.

The imaged visual object field diameter depends on the objective magnification and the eyepiece field of view (FOV) number (mm):

Imaged visual object field diameter (mm) = FOV number eyepiece (mm)/objective magnification

Simpler achromatic microscope objectives for routine work have a less flattened image diameter compared to research grade objective types (e.g. semi- apochromats or apochromats) and should therefore be used with eyepieces of correspondingly lower field of view numbers (e.g. 20-22).
When acquiring digital images, the field stop diaphragm diameter can be aligned differently, as the camera sensor size usually is considerably smaller compared to the imaged visual object field diameter. For optimum false light reduction, it is often recommended to match the field stop diaphragm diameter to the camera sensor size, by closing its diameter more than for the visual alignment. Especially for critical brightfield applications (e.g. hematology), this is important.

To operate the tutorial, use the Field Stop Diaphragm Opening slider to adjust the opening size. For example, observe how the edges of the diaphragm disappear through the eyepieces when the opening is to its maximum. At the smallest opening size setting, only a very small area of the specimen is visible in the center of the Eyepiece View.