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A microscope is an instrument used to magnify objects so that you can distinguish greater detail within an object. A microscope can facilitate a science education for students as well as provide an opportunity for hobbyists to learn more about microorganisms and other objects that are difficult to see with the naked eye. Microscopes can also stimulate interest in science within younger learners. This section on how to choose a microscope will arm you with microscope knowledge and empower you to make an informed purchase decision. We will discuss the different components of the microscope, explain the differences between the two most common microscopes, and finally we will tell you how to choose the appropriate microscope for the end user/application. We also have a Glossary of more than 70 terms applying to microscopes that may be helpful.
Microscope Basics
What Are the Common Components of a Microscope?
Microscopes consist of many different parts, and if you're buying one for the first time, understanding each one will help you make a good choice. Here are the primary parts that you should recognize:
- Eyepiece: Also known as ocular, brings the image into focus for the eye, can be 10x, 15x, or 20x
- Microscope head: Can be monocular (single eyepiece), binocular (two eyepieces), or trinocular (includes port for camera, 2 eyepieces and photoport)
- Objective: Collects light from sample and is located above the stage. 4x, 10x, 40x, 100x are most common magnifications. 100x objective is referred to as the oil immersion lens, immersion oil is used with this objective in order to collect light effectively
- Stage: Platform below the objective where specimen is viewed. Mechanical stage allows you to move slide right/left and forward/back
- Condenser: Focuses light on specimen, located below the stage
- Light source: Can be halogen, LED, tungsten, or fluorescent bulb, some economical microscopes have a mirror instead of light
- Coarse/fine focus: Bring image into focus, coarse focus is used to bring image in focus initially and fine focus sharpens the image
Compound vs. Stereo Microscope
It is imperative that one understands the differences between stereo and compound microscopes. Compound microscopes are used to view specimens on slides, such as a drop of pond water, bacterial smear, or human tissue. Stereo microscopes are used to view larger objects, such as leaves, twigs, stamps, and insects. Stereo microscopes mimic the way your eyes view objects and provide the user with a 3D image of the object. They also provide a larger working distance which allows you to manipulate the object with your hands, such as a circuit board.
Stereo Microscopes
3-D Observation/Larger Specimens
Nothing beats seeing a specimen in 3-D! If you are working with larger specimens, or if you wish to use a microscope to inspect parts, plants, stamps, coins, insects, rocks, fossils, or archaeological specimens, or to guide you during fine dissection, you need a stereo or dissecting microscope.
Stereo microscopes have the unique ability to see depth, which is the third dimension. This makes stereo microscopes the instruments of choice for surgeons, gemologists, electronic assemblers, denture makers and fine engravers, to name a few examples. Because two optical systems are joined on one frame, similar to binoculars, stereo microscopes must be perfectly collimated by the manufacturer, otherwise their use can be uncomfortable. Stereo microscopes have long working distance objectives to enable larger specimens to be examined.
How Do Stereo Microscopes Give a 3-D Image?
The binocular viewer alone does not make a stereo microscope capable of 3-D. Each eyepiece must capture a separate image from its own magnifying lens, known as an objective. The paired objectives work together to show slightly different views of an object to obtain a three-dimensional image, the same way human eyes work together to obtain depth perception. The eyepiece multiplies, or compounds, the objective magnification in the stereo microscope in the same manner as it does in the compound microscope.
Magnification and Field of View
For technical reasons, the magnification capabilities of standard stereo microscopes are usually limited to much less than 200x, typically 10x-40x. Additionally, they may cover a field of view almost as large as a quarter, or as small as a mustard seed.
Illumination
The illuminator design of dissecting microscopes commonly provides for incident light (light falling on the specimen) and trans illumination light passing through the specimen from a light source inside the base. There are options available such as a gooseneck/dual gooseneck and fiber optic illuminators for applications involving even illumination or illumination at specific angles.
Compound Microscopes
Smaller Specimens
This is the microscope preferred by medical laboratory technologists examining blood and urine, pathologists screening tissue specimens and smears for cancer cells, and microbiologists identifying bacteria in cultures. If you are interested in examining biological specimens at the cellular level, you need a compound microscope. Compound microscopes take their name from the calculation of magnification by compounding the power of the eyepiece by the power of the objective to get total magnification.
How Do Compound Microscopes Work?
We have to start at the bottom, to describe properly how a compound microscope works. Except for some applications in which light shines on the specimen from above, most compound microscopes use trans illumination light projected from below to pass through the condenser and then the specimen to the magnifying objective lens. The magnified image of the specimen projects through the draw-tube to the eyepiece where it is magnified another 10x. The eyepiece sends the image to our eyes where the lens projects it on our retina. It is just like a 35mm slide in a projector, or a movie projector at the cinema.
The illuminator is usually beneath the specimen, which rests on a platform called a stage. Atop the illuminator are lenses that help it transmit a cone of even, brilliant light onto another lens system called the substage condenser. Next is the sub-stage condenser. You can raise or lower this device with gears to control the point of focus of the light it has gathered from the illuminator so that it focuses sharply on the specimen. Working with the condenser is the iris diaphragm. This is a light valve that the user may open or close to control contrast and sharpness.
The objectives powerful magnifying lenses are directly above the specimen. Although a microscope is an optical system dependent on several key components, it is the high dry objective (40x) and the oil immersion (100x) objectives that are essential for much of the microscopes capability to magnify and resolve detail. The careful design and precision shaping of these lenses enable them to focus on the specimen so closely they may practically touch it. Our eyes normally cannot focus closer than 10, but some objectives can sharply focus on the specimen within a distance equal to the thickness of a sheet of writing paper. The 10x low dry and the 4x scanning objective are helpful to locate an area of interest using their larger fields of view.
The nosepiece is a rotating turret that can hold multiple objectives. It must be of great precision to enable each objection to rotate into perfect position centered above the specimen.
Rising from the illuminator base is the microscope arm. This structural component holds the platform stage, the focusing mechanism, the drawtube and nosepiece, and at the top, it has a socket into which fits the head or body that holds the viewing optics. The condenser rack-and-pinion is attached below the stage. Focusing mechanisms are usually one of two types. In one variation, the platform stage moves vertically with gears to bring it near or farther from the objective, which remains fixed in place. In the other type, the drawtube is moves vertically with the objective and nosepiece while the stage stays in place.
The viewing optics consist of the head and the oculars, or eyepieces. Binocular heads require prisms to divide the image-carrying light rays from the single objective to the two oculars. In most binocular heads, there is a comfort adjustment to bring the two eyepieces in line with distance between the users pupils. This is the interpupillary distance (IPD). The binocular viewer also includes a diopter adjustment. This allows the focusing characteristics of each ocular to match the users own eyes.
Magnification and Field of View
Typical magnifications of compound microscopes do not exceed 1000x. In the medical laboratory, the highest magnification most often used is 400x-500x, although a busy laboratory viewing bacteria will frequently examine specimens at 1000x.
The field of view is as little as 1/5th of millimeter less than 1/100th of an inch at 1000x magnification. About the largest field of you can expect with a compound microscope is 1/5thof an inch at 40x (using a 4x objective and a 10x ocular).
Special Illumination/Imaging Techniques
Aside from typical bright field compound microscopes, there are also dark field, phase contrast, differential interference contrast (DIC), polarization, immunofluorescence, and confocal microscopes, as well as combinations of these. There are also other types with very limited applications. Hobbyists and students will find that brightfiled microscopy will suit their needs.
Which Microscope Should You Select?
The first question you must ask yourself prior to purchasing a microscope is: what do you want to view? If you would like to look at coins and insects, then a stereo microscope is appropriate. If you would like to look at specimens prepared on slides, such as pond water, bacteria, or human tissue, then a compound microscope is required. Once you have determined what type of microscope you need, your budget and the level of use will help you make your final decision.
Microscopes can cost anywhere from $30 to $10,000 which means there is a wide range of optical quality and microscope designs. Here is a list of the most common end users accompanied by which characteristics one should look for in a compound microscope.
9 years and Younger
There are plenty of microscope starter kits available. These kits typically include an economical microscope with a variety of slides and collecting vials. These kits are designed to stimulate interest in young learners and are used as a starting point for life science education. A basic stereo microscope with 20x and 40x magnification is a great place to start because the student can take things from the backyard and view them under the microscope.
10 years old Through High School
This age group requires a student microscope. Student microscopes are built with the intention of being used on a part time basis. This means that the microscope might get some usage every day. A good student microscope will have a built in light, mechanical stage, at least 4x, 10x, and 40x objectives, (100x oil immersion lens is required to view structure of bacterial cells), and a binocular head to reduce eye strain. The mechanical stage is required at higher magnifications because if the user moves the slide with their fingers the specimen could easily be moved outside of the field of view. A built in light, as opposed to a mirror, is essential to provide better resolution. Pay close attention to the microscope base as it should be metal, not plastic.
Professionals
The difference between a professional microscope and a student microscope is that a professional microscope is designed for all day use in a doctor's office, veterinary clinic, or research laboratory. These microscopes cost significantly more than student microscopes and the best models are manufactured by companies who make their own glass. These companies have more control over the time and money they invest in their optics. Professional microscopes are also built on expandable platforms which allow the use of accessories for phase contrast, darkfield, and fluorescence microscopy. Some of the most popular brands are Meiji Techno and LW Scientific, both of which make high quality professional microscopes that are used around the world. A professional microscope may have an infinity optical system and higher quality objectives such as plan achromats. If you are considering purchasing a professional microscope consider the following: level of optics should be plan achromat or better, infinity optical system, availability of accessories for other microscopy techniques, robust stand, and the reputation of the manufacturer.
Top Microscope Features to Look for
Microscopes Made Overseas
Most if not all basic student microscopes are made in China. Manufacturers have the optics for their basic educational microscopes made in China. This does not mean you will receive an inferior product but like many products we purchase, many microscopes are made in China.
Cameras with Microscopes
There are several ways in which you can transmit images to a computer. Microscopes can accept DSLR cameras, c-mount cameras, and eyepiece cameras. A trinocular microscope has a third port where one would connect a c-mount adapter or a t-mount adapter. The appropriate camera would then connect to the adapter. There are also eyepiece cameras that will fit into a 23.2mm eyepiece tube or a 30mm eyepiece/trinocular tube. This allows you to easily record what you are viewing to review later.
Common Objects to View
Bacterial cells are typically between 1-10 microns in diameter (a micron is 1 millionth of a meter) and one can view them with a 100x oil immersion lens on a compound microscope. However, this is not enough to view viruses, which is almost impossible with most microscopes available to consumers. Instead, you will need an electron microscope to view viruses as they are on the order of 10-300 nanometers in diameter (a nanometer is one billionth of a meter). Stereo microscopes allow you to view the detail of objects such as insects, rocks, coins, and circuit boards.
Total Magnification
Magnification (the microscopes defining trait) is also the most misunderstood. Usually stated in diameters, magnification is the mathematical product of multiplying the power of the ocular, or eyepiece, times the power of the objective, which is the lens closest to the object being examined. Microscopes with 10x eyepieces and three objectives of 4x, 10x and 40x, yield 40x, 100x, and 400x magnification, respectively. If a specimen has a true diameter of 1.0mm, at 40x magnification, the detail will appear as if it were 40mm in diameter.
Microscope Resolution
Resolution is the ability to discriminate between two close objects to distinguish detail. As the resolution increases, the closer two tiny points may be to one another and still be distinguished as two tiny points. Magnification without resolution simply enlarges. Without the ability to discriminate more detail, it becomes empty magnification. Two close points will appear as one blurry blob when enlargement exceeds the capability of resolution. Theoretically limited by the interaction of glass and light, true resolution varies according to the quality and design of the equipment and the skill of the user.
Microscope Prices
The starting price point for a student microscope is around $150. If you purchase a microscope for less than this price you will struggle to product an image with decent resolution. You need to ask yourself who the microscope is for and how long you would like it to last. If you would like a microscope to last you child through high school and into college you can expect to spend roughly $300 to $400. Instead of focusing on how much you should spend you should start with a list of requirements based on the end user of the microscope. You can sacrifice features of the microscope to lower the cost if necessary.
