Microscope swing-out condensers explained

Most compound microscopes come equipped with a condenser – a lens or set of lenses that evenly focus light onto your specimen.

But is one condenser enough?

If you’re after the very best image quality, you’ll find that two is better.

Fortunately, you won’t have to constantly swap your whole condenser – you just need a swing-out condenser.

What is a swing-out condenser?

As the name suggests, a swing-out condenser simply has an extra condenser that you ‘swing out’ in front of the main condenser body.

It’s a bit like putting reading glasses on your microscope.

Using one is very simple – when you are using high magnification objectives, swing the secondary condenser into the light path. When using low power objectives, swing it back out of the way.

So, why do you need a secondary condenser in the first place?

Why do you need a swing-out condenser?

The reason a single all-purpose condenser isn’t the best is because of the range of numerical aperture (NA) values it has to deal with.

What is numerical aperture?

NA is a measure of the resolution of an objective lens – a higher NA lets you distinguish more detail. It captures light at sharper, more oblique angles, so it’s better at capturing light than a lower NA lens.  

Lenses with different NA values need different amounts of light to be let through the condenser to produce the best image quality.

Dealing with different NA values

Ideally, the iris of your condenser should be set to match the NA value of your current objective, letting the appropriate amount of light for that NA through.

The problem is that the appropriate cones of light vary wildly in size. Microscopes generally come with 4 different objective lenses, each with their own very different NA values.

For example, the NA of 0.10 for a 4x objective might require a light cone with a diameter of 8mm, while the 1.25 NA of a 100x objective might call for a 0.2mm cone of light.

Double the condenser, half the problem

The issue then is that a single condenser must try to accommodate big differences in light cone diameter.

If it’s too much for one condenser to handle, why not use two?

For this reason, the swing-out condenser was created – two different condensers combined into one, able to swing in and out of position as needed. Swung in for high power, swung out for low power.

By having the different condensers handle either the upper or lower range of NA values, the range of light cone diameters they have to handle is drastically reduced.

Each condenser can provide the perfect amount of light for its designated NA range, completely filling the field of view as required.

So, if you’re looking for the best possible image quality, consider investing in a swing-out condenser.

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