Abbe Condenser Focusing Method for Microscopes Without Iris Diaphragm

Numerical Aperture

Numerical aperture is defined as the range of angles over which the objective can accept light and is expressed numerically as:

Where n is the refractive index of the medium (~ 1 for air) and θ is half-angle of the maximum cone of light the objective can accept. For maximum resolving power:

This ensures the specimen is illuminated from all angles the objective can use but it doesn’t always produce the optimal image for visual examination as it decreases contrast.

Microscopists therefore reduce the cone of illumination slightly to produce stronger contrast and more visible structure and a classic rule taught in teaching labs was that the illumination cone should fill approximately 70-80% of the objective aperture.

Focusing Method

The goal is to focus the condenser so the illumination cone properly fills the objective aperture and with a fixed iris condenser such as the type on this instrument the height of the condenser becomes the main control. The procedure is as follows:

1. Place the specimen on the stage and focus it with the objective as usual
2. Lower the condenser to the bottom of its travel – the expectation is that at this point the field will look dim, flat and of low contrast
3. Slowly raise the condense while looking through the eyepiece and look for the “snap”, the point where the image suddenly becomes brighter, higher contrast and with sharper edges

At this point, the condenser is properly focusing the illumination into the objective and the cone of light produced by the condenser almost matches the numerical aperture of the objective.

When adjusting the condenser using this method, the cone of light entering the objective is initially too narrow but expands as it approaches the objective NA, producing the “snap point”. Beyond that point, the illumination overfills the objective producing glare and a reduction in contrast.