Condenser and Iris Diaphragm
The two controls that decide whether you can see anything.
These sit below the stage and are the most misused parts of the instrument. Together they shape the cone of light reaching the specimen, which sets both contrast and the resolution you actually achieve.
The condenser
The condenser gathers light from the illuminator and focuses it into a cone converging at the specimen plane. Without it, light arrives as a weak near-parallel beam and the objective's numerical aperture goes unused. The condenser has its own NA, and it should be matched to the objective in use — a 1.25 NA oil objective cannot deliver 1.25 NA performance if the condenser below it is supplying a 0.3 cone.
The iris diaphragm
The iris diaphragm is an adjustable aperture in the condenser that changes the angle of the illuminating cone.
- Open — wide cone, maximum resolution, minimum contrast. Unstained specimens can disappear entirely.
- Closed down — narrow cone, higher contrast, reduced resolution, and diffraction artefacts appear as false outlines around edges.
The working setting is a compromise, usually with the diaphragm at roughly 70–80% of the objective's NA. Adjust it every time you change objective.
Brightness is a different control
Use the lamp rheostat or a neutral density filter for brightness. Using the iris to dim the image throws away resolution to solve a problem the light control already solves. This is the single most common bad habit in teaching labs.
Field diaphragm and Köhler illumination
On stands with a field diaphragm, Köhler illumination gives even lighting and the best contrast: focus the specimen, close the field diaphragm, raise or lower the condenser until the diaphragm edge is sharp, centre it, then open it just past the field of view. Set the iris diaphragm last.