Microscope condenser and aperture diaphragm explained

If you’re interested in making the most of your microscope, you’ll want to get to grips with the condenser and aperture diaphragm on your scope.

These essential components play a critical role in the quality of your images – and you’ll need to understand a little of their nuances to get a sharp, well-contrasted look at your samples.

Where can you find your condenser, and what does it do?

The condenser is a lens or series of lenses that usually sits just below the microscope stage. Its role is to focus the light into an even illumination to give a clear and undistorted view of your samples.

The amount of light that passes through the condenser is controlled by the aperture diaphragm.

Where is the aperture diaphragm, and what does it do?

The aperture diaphragm, also known as the iris or iris diaphragm, consists of a circle of interlocking petals or blades that sits within the condenser. It functions a bit like curtains, controlling the amount of light that gets through the condenser.

You can increase or decrease the size of the aperture that the light passes through using a simple lever on the side of the condenser.

Why are they so important?

Together, these elements control the cone of light that reaches your sample.

While it sounds like they simply act as another illuminator control or a glorified dimmer switch, they actually have a massive impact on the quality of your images.

Collectively, they determine resolution, contrast and depth of field. The position of that little lever decides whether you’ll see a washed-out mess or a crisp, detailed specimen.

Adjusting your aperture diaphragm

While the aperture diaphragm is controlled by a simple lever, there’s an art to it.

Closing the diaphragm will increase the contrast and depth of field, but at the expense of resolution and brightness. Opening it up will do the opposite.

You can’t have it all, so it’s about finding the right balance for the specimen at hand. A rule of thumb is to have the diaphragm 50 to 90% open for a good mix of resolution and contrast.

If you’re looking to maximise resolution, match the aperture setting (the numbers below the control lever) to the numerical aperture (NA) value of the objective lens you’re using.

But note that you can’t increase the resolution beyond the NA value, so opening the diaphragm any further won’t give you any additional benefits.

Many novice users are tempted to close the iris and crank up the illuminator setting for a bright, yet highly contrasted image. But hold on a sec!

While it can be helpful, it can also introduce a few optical issues. Dust and debris on the cover slip or optical surfaces can become more visible and obstruct the view, and specimen structures can overlap or become hidden.

Everyone’s preferences and specimens are different, so don’t be afraid to play around with the settings until you find that sweet spot!