The most common cause of disappointing results is poor setting up of
the microscope before use. The performance of almost all microscopes can
be improved if a little time is spent focusing before use. No matter
what quality microscope you buy, it makes sense to get the best possible
image it is capable of.
An often neglected part of the microscope is the substage condenser,
found on all but the most basic of instruments. The condenser focuses
and concentrates the light uniformly onto the specimen. Most
importantly, because it controls the size of the cone of light
illuminating the stage. It also controls resolution - i.e. how sharp or
fuzzy the image is.
Ideal illumination is obtained by critical focusing which ensures
that the specimen and light source are properly centred and focused,
with just the right amount of light to give a clear, uniformly bright
Setting up for critical focusing
Put a prepared slide on the stage and bring
it into focus with the 10x or 20x objective
Next focus the condenser. How this is done depends on whether light
source has a field diaphragm such as found with Köhler-type
illumination. Köhler lighting systems have an iris or field diaphragm
which controls the aperture of light going into the condenser.
Although this form of illumination is gaining it popularity, it is would
not normally be found on the average hobbyist microscope.
Focusing the condenser on a microscope without an iris diaphragm is
carried out by removing the slide from the stage and placing a piece of
thin card half-way across the light source aperture. Adjust the
condenser - not the stage or objective - by racking it up or down until
the card is in sharp focus when viewed through the eyepiece lens. This
usually occurs just as the light interference halo turns from blue to
At this point the condenser is properly focused and should not need
to be adjusted again. With Köhler lighting systems focusing is carried
out in exactly the same way - only focusing on the leaves of the
diaphragm rather than a piece of card.
Centering the condenser
Next, it is important that the condenser is centred or it will focus
the light at some point to the side of the specimen. To centre the
condenser close down the iris diaphragm and remove the eyepiece.
Looking down the viewing tube you will see a small point of light (see
diagrams). Make sure that the light appears in the centre of the tube by
adjusting the condenser positioning screws. This takes a bit of fiddling
about - but is simple once you are used to it.
with the eyepiece removed, look down the
viewing tube when you will see something like the diagram on the
left - an off-centre light spot. Adjust the condenser centering
screws until the light is in the middle
The iris diaphragm
The final step is to adjust the substage or iris diaphragm. This is
done with a lever or screw found on the condenser which works in a
similar fashion to that of the iris of a camera. As the iris is opened
it allows more light into the condenser. The iris diaphragm is often
used incorrectly to control light intensity. While this might seem
logical, its proper use is to control the size of the cone of light
entering the objective lens. The correct iris diaphragm setting varies
with each objective. Consequently it needs to be re-adjusted every time
you change magnifications
To set up the iris diaphragm look down the viewing tube without the
eyepiece in place and slowly open up the iris diaphragm until the circle
of light just about fills the viewing tube. Finally replace the
eyepiece, re-focus and adjust the light intensity if your microscope has
Once the light spot is central, open up the
condenser iris until the light just fills the field of view.
Replace the eyepiece
Provided that your lens and slide are clean you should now get the
best possible image.