Posted by Fred Koenig on Oct 14th 2022

What Does the Objective Lens Do on a Microscope

A magnifying glass uses a convex lens to magnify an image or make it appear larger so you can see it better with the human eye. It’s actually the most basic form of a magnifying glass, and it’s an easy-to-understand representation of an objective lens.

How Does a Magnifying Lens Work

Taking the example of a magnifying glass, the convex lens is held between the human eye and the subject. To see the subject more clearly, you simply move your hand closer to it or further away. The glass in a magnifying lens is convex, which means it curves outward. This outward curve bends light rays that bounce off the subject and bring them back together, creating a virtual image on the back of your retina, so your eyes see the object as larger than it is.

It's believed that the first magnifying glass was created by a scientist named Alhazen in 1021. That invention opened people’s eyes to things they never knew existed before because they couldn’t see them. It’s quite remarkable to imagine people first realizing that the world around them consists of things they cannot see.

From the magnifying lens and the use of this objective lens came other inventions, including binoculars, cameras, telescopes, microscopes, and more.

What Is the Objective Lens on a Microscope

In a microscope, the  objective lens has much the same function as it does in a magnifying lens. That’s why it’s useful to understand how a magnifying lens works. The very first microscopes are called compound microscopes because they used two lenses. Modern compound microscopes may use two or more lenses.

The Janssen microscope was invented in the late 1500s and consisted of an objective lens at one end of a tube and an eyepiece at the other end. The objective lens was placed above the subject, and the viewer put their eye up to the eyepiece to look through the tube. These two lenses worked in tandem to amplify the image and transmit it to the human viewer’s eye.

What Are Modern Microscope Objectives

Today, we’ve moved way beyond the original Janssen microscope. Objectives in modern microscopes provide real, magnified images from 2x to 200x. They’re much more complicated than the simple convex lens of the past, but their function is basically the same. There are two main types of objectives to choose from, refractive and reflective.

Refractive Objectives

Traditional objectives were refractive in nature. The light is passed through the system and bent by the optical elements. The original microscope used the convex lens closest to the subject to do this. Today’s objectives can use multiple lenses, each with a different task, to achieve the clearest resolution with little to no aberration.

Reflective Objectives

Reflective objectives use a mirror-based design to magnify and relay the image of the subject. Using mirrors rather than glass, you get higher light efficiency and better resolving power for exceptionally fine details.

Elements of an Objective

Think of the modern objective like a fancy camera lens – which is a photographic objective. It has several working parts within it that provide a better view of the subject matter. In the typical modern microscope objective, different elements are typically printed on the body for easy identification. Look for these following notations:

  • Application: This tells you what application is best suited for this objective.
  • Magnification: The magnification is printed on the objective body and often repeated by the use of a color-coded ring that easily lets the user know what level of magnification they have.
  • Numerical aperture/immersion medium: This is often referred to as the NA for numerical aperture. Large NA objectives sometimes require immersion mediums, which may also be listed on the objective.
  • Special design properties: Microscopes use DIN or JIS standards. DIN is 160mm distance from the objective flange to the eyepiece flange, and a JIS system uses 170mm distance.
  • Lens image distance/coverslip thickness: A coverslip is sometimes used to protect the subject under inspection and the microscope components from contamination. Adding that protective layer of glass changes the way the light refracts, and this piece of data helps the user correct for variations in light refraction.
  • Working distance: The working distance defines the free space between the object and the leading edge of the lens.
  • Color-coded magnification identifying ring: There will be a colored ring around the objective which adheres to a standard magnification key to let you know what magnification the objective has.

Understanding the elements of the modern objective can help you to select the appropriate tool for your applications. We’ve certainly come a long way since the basic magnifying glass and the original compound microscopes, but an objective is still the lens or series of lenses used to begin the magnification process of a subject.