Understanding microscope objective lenses

Objective lenses – what lies inside?

Objective lenses look quite simple from the outside. A small tube with a glass lens running through it, perhaps? No, that plain exterior houses an elaborate array of precisely crafted lenses that determine your microscope’s performance.

Here’s a picture of what’s inside your objective lenses.

The internal lenses

What looks from the outside like a single lens may in fact be a single lens. Simpler objectives can indeed use only one or 2 lenses. More advanced objectives, however, can house intricate arrangements of more than a dozen lenses.

These complex arrays provide better optical quality and can help increase your field of view, colour correction and numerical aperture values.

The exact arrangement depends on the model and manufacturer, but the types of lenses used are largely the same – triplets, doublets, singles, hemispherical and meniscus lenses.

Triplets

As the name suggests, triplet lenses are comprised of 3 different lenses. This 3-in-1 arrangement is used to combat particular kinds of optical aberrations.

The lenses can either be fused together, reducing any air-related distortion, or kept separate, which gives the manufacturer greater freedom when trying to compensate for any aberration.  

Doublets

The aptly named doublet is a pair of lenses generally used to reduce chromatic aberration and, to a lesser extent, spherical and optical aberrations.

The lenses are usually made of 2 different types of glass with different refractive indexes and light dispersion qualities. The dispersion from the first lens is corrected by the second lens, which helps reduce colour distortion.

Similar to triplets, doublet lenses can be separated by an air gap, stuck together, or sometimes held together by the surface tension of optical oil.

Singles

Just a regular lens, striking out on its own.

Hemispherical lens

This lens, shaped like a half sphere, is used to collect and gather light. As such, it is often found on the front of the lens facing the specimen.

Meniscus lens

A meniscus lens has two curved surfaces – one convex, the other concave. By itself, it can help reduce spherical aberration, but when used in conjunction with another lens, such as a hemispherical lens, it can help increase the numerical aperture value.

Other internal components

To help support and hold the lens arrangements in place, you might find some other components inside an objective lens.

Spacers

Lenses are often separated by ring-like lens spacers.

Springs

Some objectives have spring-loaded arrangements that help protect the lenses from damage.

Correction collar adjuster

To compensate for unusual coverslip thicknesses, some lenses come equipped with a ‘correction collar’. This is usually visible on the exterior as an adjustable corrugated ring halfway along the barrel.

By adjusting the ring on the outside, some of the internal lenses can be moved up and down to compensate for the various aberrations that non-standard coverslips can introduce.