When you are using the high power objective lens of the microscope which part of the microscope will you use to focus the image?

Historians credit the invention of the compound microscope to the Dutch spectacle maker, Zacharias Janssen, around the year 1590 (more history here). The compound microscope uses lenses and light to enlarge the image and is also called an optical or light microscope (versus an electron microscope). The simplest optical microscope is the magnifying glass and is good to about ten times (10x) magnification.

The compound microscope has two systems of lenses for greater magnification:

1. Ocular eyepiece lens to look through.
2. Objective lens, closest to the object. Before purchasing or using a compound microscope, it is important to know the functions of each part. This information is presented below. Links will take you to additional information and images.

The Functions & Parts of a Microscope

Eyepiece Lens: the lens at the top that you look through, usually 10x or 15x power.

Tube: Connects the eyepiece to the objective lenses.

Arm: Supports the tube and connects it to the base.

Base: The bottom of the microscope, used for support.

Illuminator: A steady light source (110 volts) used in place of a mirror. If your microscope has a mirror, it is used to reflect light from an external light source up through the bottom of the stage.

Stage with Stage Clips: The flat platform where you place your slides. Stage clips hold the slides in place. If your microscope has a mechanical stage, you will be able to move the slide around by turning two knobs. One moves it left and right, the other moves it up and down.

Revolving Nosepiece or Turret: This is the part of the microscope that holds two or more objective lenses and can be rotated to easily change power.

Objective Lenses: Usually you will find 3 or 4 objective lenses on a microscope. They almost always consist of 4x, 10x, 40x and 100x powers. When coupled with a 10x (most common) eyepiece lens, total magnification is 40x (4x times 10x), 100x , 400x and 1000x. To have good resolution at 1000x, you will need a relatively sophisticated microscope with an Abbe condenser. An Abbe condenser is composed of two lenses that control the light that passes through the specimen before entering the objective lens on the microscope. The shortest lens is the lowest power, the longest one is the lens with the greatest power. Lenses are color coded and if built to DIN standards are interchangeable between microscopes. "DIN" is an abbreviation of "Deutsche Industrial Normen". This is a German standard that has been adopted internationally as an optical standard used in most quality microscopes. A typical DIN standard microscope objective lens has a 0.7965" (20.1mm) diameter threads, 36 TPI (threads per inch), and a 55º Whitworth. Many high power objective lenses are retractable (i.e. 40XR). This means that if they hit a slide, the end of the lens will push in (spring loaded) thereby protecting the lens and the slide. All good quality microscopes have achromatic, parcentered, parfocal lenses.

Condenser Lens: The purpose of the condenser lens is to focus the light onto the specimen. Condenser lenses are most useful at the highest powers (400x and above). Microscopes with in-stage condenser lenses render a sharper image than those with no lens (at 400x). If your microscope has a maximum power of 400x, you will get the maximum benefit by using a condenser lenses rated at 0.65 NA or greater. 0.65 NA condenser lenses may be mounted in the stage and work quite well. A big advantage to a stage mounted lens is that there is one less focusing item to deal with. If you go to 1000x then you should have a condenser lens with an N.A. of 1.25 or greater. All of our 1000x microscopes use 1.25 Abbe condenser lens systems. The Abbe condenser lens can be moved up and down. It is set very close to the slide at 1000x and moved further away at the lower powers.

Diaphragm or Iris: Many microscopes have a rotating disk under the stage. This diaphragm has different sized holes and is used to vary the intensity and size of the cone of light that is projected upward into the slide. There is no set rule regarding which setting to use for a particular power. Rather, the setting is a function of the transparency of the specimen, the degree of contrast you desire and the particular objective lens in use.

How to Focus Your Microscope: The proper way to focus a microscope is to start with the lowest power objective lens first and while looking from the side, crank the lens down as close to the specimen as possible without touching it. Now, look through the eyepiece lens and focus upward only until the image is sharp. If you can't get it in focus, repeat the process again. Once the image is sharp with the low power lens, you should be able to simply click in the next power lens and do minor adjustments with the focus knob. If your microscope has a fine focus adjustment, turning it a bit should be all that's necessary. Continue with subsequent objective lenses and fine focus each time.

If you are unsure of the parts and functions of your microscope, contact Microscope World.

This page has activities and free printouts for labeling parts of the microscope.

Related Articles:

Microscope Objective Lenses

Types of Microscopes

Infographic on the History of the Microscope

Familiarization  First, familiarize yourself with all the parts of a microscope so that you can easily move from one part to another during operation. 

Moving and Placement: It is worth remembering that while a good quality microscope will last a lifetime, it is a sensitive scientific instrument that will suffer damage from sharp blows or impact. Always, therefore, carry your microscope in both hands. Grasp the arm with one hand and place the other hand under the base for support. Always place the microscope on a level and stable surface.

Slide Preparation: Microscope slides should always be prepared with a cover slip or cover glass over the specimen. This will help protect the objective lenses if they touch the slide. To hold the slide on the stage fasten it with the stage clips. You can push down on the back end of the stage clip to open it.

Focusing the Microscope:

•     Start by turning the revolving nosepiece (turret) so that the lowest power objective lens is "clicked" into position. The lowest power objective is the shortest one. This objective is the easiest to focus and center the image in the field of view.
•     While looking at the objective lens and the stage from the side, turn the coarse focus knob so that the stage moves upward toward the objectives. Move it as far as it will go without touching the slide.
•     Now, look through the eyepiece(s) and adjust the illuminator and diaphragm until you attain the maximum, comfortable level of light.
•     Slowly turn the coarse adjustment so that the stage moves down (away from the slide). Continue until the image comes into broad focus. The turn the fine adjustment knob, as necessary, for perfect focus.
•     Move the microscope slide until the image is in the center of the field of view. Then readjust the illuminator or diaphragm in order to attain the clearest image.
•     Once you have attained a clear image, you should be able to change to a higher power objective lens with only minimal use of the focusing adjustment. If you cannot focus on your specimen, repeat the above steps and work from objective to objective until the higher power objective lens is in place.

General Advice: You should check to see if your microscope has a rack stop. If it does not have one, then be careful not to allow the objective lens to touch the slide as you may break the slide. When using a monocular microscope, the correct technique is to look through the eyepiece with one eye and keep the other eye open. Most new users, tend to close one eye. While many microscopists do close one eye, you will help avoid eye strain by keeping both eyes open. Finally, remember! When you view a specimen through a microscope, you are viewing an image through multiple lenses. As a result, the image is upside down and back-to-front so when you move the slide to the right, the image moves to the left and vice versa!

Care & Maintenance of Your Microscope: Your compound microscope will last a lifetime if cared for properly and we recommend that you observe the following basic steps:

•     When finished viewing , lower the stage, click the low power lens into position and remove the slide.
•     Switch off the microscope when not using.
•     Avoid touching the glass part of the lenses with your fingers. Use only special lens paper to clean the lenses.
•     Dust is the number one enemy of a microscope so always keep your microscope covered when not in use. When not in use for extended periods, replace the microscope in its box.