This article is based on material originally published in Illustrated Guide to Home Chemistry Experiments: All Lab, No Lecture, by Robert Bruce Thompson, and the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement, by Robert Bruce Thompson and Barbara Fritchman Thompson.
Without a microscope, we are limited to what we can see with the naked eye. Using a microscope reveals entire worlds that would otherwise be invisible to us. Obviously, a microscope is essential for the serious study of biology and forensics. Less obviously, a microscope is also an important tool in disciplines as diverse as chemistry, Earth science, and physics.
Every home scientist should make it a high priority to acquire a good microscope. The question is, which one? This article explains what you need to know to choose a microscope appropriate for your needs and budget.
Price
First, let’s talk about price. Microscopes are available in an incredible range of prices, from $25 toy microscopes to professional models from German and Japanese manufacturers that can cost as much as a new Mercedes-Benz automobile. Literally. Toy models are obviously unsuitable for serious use, but few of our readers will have the inclination (or budget) to spend thousands on a professional model. Fortunately, there’s a happy middle ground of inexpensive, high-quality microscopes that sell in the $150 to $1,200 range. We’ll focus on that category.
All of these microscopes are Chinese-made. The best of the Chinese microscopes are very good, both optically and mechanically. Unfortunately, Chinese factories also produce boatloads of garbage microscopes, and it’s impossible to tell the difference just by looking at the scopes or comparing prices. The best way to get a good one is to buy from a reputable dealer.
Microscope Types
Broadly speaking, two types of microscopes are useful in home science labs. A compound microscope, shown in Figure 1, is what most people think of as a microscope. You use it to view small specimens by transmitted light at three or four medium to high magnifications, typically 40X, 100X, 400X, and sometimes 1000X. A good compound microscope is essential for serious study of biology or forensics, and useful for many other sciences.
Figure 1. A typical compound microscope (image courtesy National Optical & Scientific Instruments, Inc.)
A stereo microscope, shown in Figure 2, uses two eyepieces, each with its own objective lens, to provide a 3D image of the specimen. A stereo microscope (also called a dissecting microscope or an inspection microscope) operates at low magnifications, usually in the 10X to 50X range. Some models have fixed magnification, usually 10X, 15X, or 20X. Other models offer a choice of two magnifications, often 10X or 15X and 30X or 40X. Zoom models offer continuously variable magnification.
Figure 2. A typical stereo microscope (image courtesy National Optical & Scientific Instruments, Inc.)
A stereo microscope is useful for examining relatively large solid objects at low magnification by reflected rather than transmitted light. Most stereo microscopes provide a top illuminator that directs light downward onto the specimen. Better models often also offer a bottom illuminator that allows specimens to be viewed by transmitted light.
For a home lab, a stereo microscope is useful but not essential. Buy one if you can afford it, but don’t skimp on the compound microscope. It’s better to buy a good compound microscope and no stereo microscope than to buy cheap models of each. If you don’t have a stereo microscope, you can substitute a magnifier or pocket microscope, or in some cases simply use your compound microscope at its lowest magnification.
Head Style
Compound microscopes may be available in any or all of the four head styles shown in Figure 3.
- A monocular head provides only one eyepiece. This is the least expensive of the four head styles, and is suitable for general use.
- A dual head provides two eyepieces, one vertical and one angled. The second eyepiece allows two people to view a specimen simultaneously, for example a teacher and a student. A dual head is also very convenient if you want to mount a still or video camera to image specimens. Dual head models typically cost $50 to $100 more than comparable monocular models.
- A binocular head provides two eyepieces to allow viewing specimens with both eyes. One eyepiece is individually focusable to allow the instrument to be set up for one person’s vision. The advantage of a binocular head is that it’s less tiring to use over long periods and may allow seeing more detail in specimens. The disadvantage is that the focusable eyepiece must be adjusted each time a different person wants to use the scope. Binocular models typically cost $150 to $250 more than comparable monocular models.
- A trinocular head provides two eyepieces for binocular viewing and a separate single eyepiece for viewing by a second person or for mounting a camera. Trinocular models typically cost $300 to $400 more than comparable monocular models.
At any particular price point, a monocular-head model offers the maximum bang for the buck. You’ll get better optical and mechanical quality with the monocular head than with any of the multiple-head models.
Figure 3. Monocular, dual-head, binocular, and trinocular head styles (images courtesy National Optical & Scientific Instruments, Inc.)
Regardless of head style, most better models allow the head to be rotated through 360° to whatever viewing position you prefer. The left image in Figure 3 shows the traditional viewing position, with the support arm between the user and the stage. The other three images show the reversed viewing position, with the stage between the user and the support arm. Most people prefer the latter position, which makes it easier to manipulate slides, change objectives, and so on.
Illumination Type and Power Source
Early microscopes and some inexpensive current models have no built-in illuminator. Instead, they use a mirror to direct daylight or artificial light up through the stage and into the objective lens. Because any mirror small enough to fit under the microscope stage gathers insufficient light to provide bright images at high magnifications, such scopes are limited to use at low and medium magnifications unless they are equipped with an accessory illuminator. Most microscopes include built-in illuminators of one of the following types, roughly in order of increasing desirability:
- Tungsten – the least expensive method, and the most common on low-end scopes, tungsten illuminators use standard incandescent light bulbs. They are relatively bright, but they produce a yellowish light and considerable heat. In particular as the light is dimmed, it shifts further toward orange. This warm color balance can obscure the true colors of specimens. The heat produced by the incandescent bulb may kill live specimens and quickly dries out temporary wet mounts made with water. Lamp life is relatively short.
- Fluorescent – costs a bit more than tungsten, and was quite popular before the advent of LED illuminators. Fluorescent illuminators provide bright light that appears white to the human eye, but is actually made up of several different discrete colors that are mixed to appear white. Accordingly, color rendition can differ significantly from the true color rendition provided by daylight. Fluorescent bulbs emit much less heat than incandescent bulbs, and so are well suited to observing live specimens. Some fluorescent illuminators are battery-powered, but most use AC power. Lamp life is relatively long.
- LED – priced about the same as fluorescent illuminators, LED illuminators have become very popular, largely replacing fluorescent illuminators. LED illuminators have the same color-rendition problems as fluorescent illuminators, but are otherwise ideal for many purposes. LED illuminators draw very little power and emit essentially no heat. Their low power draw means they’re the best choice for a battery-powered microscope, and are ideally suited for portable microscopes that can be used in the field. Lamp life is essentially unlimited.
- Quartz-halogen – the most expensive type of illuminator, and the one preferred by most microscopists. They provide a brilliant white light needed for work at high magnification that reveals the true colors of specimens. Unfortunately, quartz-halogen lamps also produce more heat than any other type of illuminator. Their high power draw means they are AC-only. Lamp life is relatively short.
Choose quartz-halogen if it is available for the scope model you purchase. Otherwise, choose LED. Tungsten is appropriate only for an entry-level scope.
Nosepiece, Objectives, and Ocular (Eyepiece)
The nosepiece, also called the turret, is a rotating assembly that holds 3, 4, or (rarely) 5 objective lenses. By rotating the nosepiece, you can bring any a different objective lens (usually just called an objective) into position and change the magnification you use to view the specimen. Inexpensive microscopes use friction-bearing nosepieces; better models use ball-bearing nosepieces with positive click-stop detents. Figure 4 shows a typical nosepiece with three objectives visible.
Figure 4. A typical microscope nosepiece with objective lenses
The nosepiece may be mounted in the forward position (tilted away from the support arm) or reverse position. If you use the scope in the forward viewing position (with the support arm between you and the stage), having the nosepiece mounted in the forward position makes it a bit easier to change objectives. If you use the reverse viewing position, it’s easier to use a nosepiece mounted in the reverse position.
Objective lenses are usually color-coded to make it obvious which one is currently being used. The standard color codes are red (4X), yellow (10X), green (20X), light blue (40X or 60X), and white (100X). Not all manufacturers follow this standard.
Inexpensive microscopes usually provide three objective lenses, 4X, 10X, and 40X. Better microscopes usually include a fourth, 100X, objective lens. The overall magnification of the microscope is the product of the objective lens magnification factor and the eyepiece (ocular) magnification factor. So, for example, if your microscope has a 10X eyepiece and 4X, 10X, and 40X objectives, your available magnifications are 40X, 100X, and 400X. If you also have a 100X objective, you also have 1000X magnification available. If you replace the standard 10X eyepiece with a 15X eyepiece, your available magnifications become 60X, 150X, 600X, and 1500X, which is about the maximum magnification usable with an optical microscope.
Microscope objective lenses differ in two major respects, color correction and flatness of field.
Color correction
The level of color correction is specified as either achromatic or apochromatic. Achromatic lenses are corrected for chromatic aberration at two specific wavelengths of light, usually red and green. An achromat brings those two wavelengths to the same focus, with other wavelengths very slightly out of focus. An apochromat is corrected for three specific wavelengths of light–usually red, green, and blue–and brings those three wavelengths to the same focus, providing slightly sharper images than an achromat. Apochromatic objectives are extremely expensive, some costing more than $10,000, and are found only on professional-grade microscopes. Any microscope affordable for a home lab uses achromatic objectives.
Flatness of field
Standard objectives have limited correction for spherical aberration, which means that only the central 60% to 70% of the field of view is in acceptably sharp focus. Semi-plan objectives have additional correction that extends the sharp focus area to the central 75% to 90% of the field of view. Plan objectives extend the area of sharp focus to 90% or more of the field. This additional correction for flatness of field is completely independent of color correction. You can, for example, buy semi-plan apochromatic objectives and plan achromat objectives.
Finally, some vendors offer optional upgrades to superior lens coatings, often under such names as Super High Contrast or something similar. These superior coatings don’t improve color correction or flatness of field, but they increase image contrast noticeably.
For most home lab use, ordinary achromatic objectives provide perfectly acceptable images and are by far the least expensive choice. My own microscope, a Model 161 dual-head unit shown in Figure 3, has the upgraded ASC objectives, which I purchased because I planned to do a lot of photography through the microscope. Otherwise, I’d have bought the standard achromatic objectives.
Parfocality and Parcentrality
All but toy microscopes are parfocal and parcentered. Parfocal means that all objectives have the same focus. When you focus a specimen at 40X, for example, and then change to 100X, the specimen remains focused. (You may have to touch up the focus with the fine-focus knob, but the focus should be very close to start with.) Parcentered means that if you have an object centered in the field of view with one objective and you change to a different objective, the object remains centered in the field of view. Professional-grade microscopes provide adjustments for both parfocality and parcentrality, but student- and hobbyist-grade microscopes are set at the factory and cannot be adjusted by the user. That means it’s important to check these settings as soon as you open the box of your new microscope.To check parfocality, place a flat specimen (a thin-section or smear slide is good, if you have one; otherwise any flat specimen) on the stage and focus critically on it at the lowest magnification. Then change to your next highest magnification and check the focus. It should be in focus or nearly so, requiring at most a partial turn of the fine-focus knob to bring it into critical focus. Change to your next higher magnification and again check the focus. Again, it should require at most a small tweak with the fine-focus knob to bring the specimen into sharp focus.
To check parcentrality, center an object in the field of view at the lowest magnification and then switch objectives to the next-higher magnification. The object should remain centered, or nearly so. Repeat until you are viewing the object at your highest magnification. Because it’s easier to judge whether an object is centered at high magnification, center the object at your highest magnification and then work your way down to lower magnifications. If the object remains centered (or nearly so), your parcentrality is acceptable. If the position of the object in the field of view shifts dramatically when you change objectives, the parcentrality is off. The only solution is to return the microscope for a replacement. (All of the scopes sold by Maker Shed are manually checked for parfocality and parcentrality before shipping and should be fine unless they are damaged in shipping, which very rarely happens.)
The ocular (or eyepiece) magnifies and focuses the image provided by the objective lens and presents it to your eye. Standard microscope ocular barrels are either 23.2 mm (usually abbreviated to 23 mm) or 30 mm in diameter, which means it’s easy to exchange oculars if you need a different magnification range. The standard ocular magnification factor is 10X, but 15X oculars are readily available to increase the range of magnifications available to you. Avoid zoom oculars, which invariably produce inferior images.
Most toy microscopes have single-element oculars, sometimes made of plastic, that provide a distorted, dim, narrow view. Better microscopes, including all of the models offered by Maker Shed, provide multi-element optical glass oculars that provide a flat, bright, wide field of view with minimal distortion.
Most standard oculars are unobstructed, but some have a standard or optional pointer or reticle (grid or graduated scale). A pointer is primarily useful in a teaching or collaborative environment, where one person can place the pointer on an object of interest so that the other person can identify it unambiguously. A graduated reticle is useful in biology and forensics for measuring the size of objects in the field of view, and a grid reticle is useful for counting large numbers of small objects in the field of view.
Focusing
Microscopes use one of two methods for focusing. Most older models and some current models keep the stage in fixed position and move the head up and down to achieve focus. Most current models and some older models reverse this, keeping the head in a fixed position and moving the stage up and down to achieve focus. Either method works well.
Toy microscopes and the least expensive hobby/school models have a single focus knob that changes focus at intermediate rate, which makes it difficult to achieve critical focus. Midrange models have separate coarse-focus and fine-focus knobs. More expensive models usually have a coaxial focus knob, often one one each side of the microscope, with the coarse focus on the outer knob and the fine focus on the inner knob, as shown in Figure 5.
Figure 5. Coaxial focusing knob, with coarse focus (outer ring) and fine focus
You use the coarse-focus knob to bring the specimen into reasonably close focus, and then use the fine-focus knob to tweak the focus slightly to achieve the sharpest possible focus. If you are viewing a three-dimensional object, particularly at higher magnifications, you’ll find that you can’t bring the entire depth of the object into focus at the same time. You use the fine-focus knob to adjust focus slightly as you’re viewing the object to view different “slices” of it in depth.
Many coaxial focus knobs, including the one in Figure 5, provide a graduated scale. One obvious use for this scale is in a collaborative situation. One person can focus critically, note the scale setting, and then turn over the microscope to the second person, who refocuses as necessary. When the first person returns to the eyepiece, merely resetting the scale to the original value puts the specimen back into critical focus. A less obvious use of the graduated scale is to determine relative depths of parts of a specimen. By setting a baseline focus on one level of the specimen and then noting how much change in scale units is needed to refocus on parts of the specimen at different depths, you can get a relative idea of the differences in depth of different parts of the specimen.
Mechanical Stage
Inexpensive microscopes use a pair of clips to secure the microscope slide to the stage. Although workable at low magnifications, this method becomes increasingly difficult as you increase magnification. The problem is that a very small movement of the microscope slide translates into a huge movement in the field of view. At low magnification, the smallest movement you can make manually may move an object from one side of the field of view to the other. At higher magnifications, the smallest movement you can make manually may move the object completely out of the field of view. If you’re viewing a living, moving object (such as a paramecium), it can be almost impossible to keep the object in the field of view.
The solution to this problem is a mechanical stage, shown in Figure 6. With a mechanical stage, you clamp the slide into an assembly that provides rack-and-pinion gearing that allows you to turn knobs to move the slide continuously along the X-axis (left or right) and the Y-axis (toward or away from you) in extremely small small increments.
Figure 6. A typical mechanical stage (note the verniers on the X and Y axes and the top lens of the Abbe condenser below the stage)
Centering an object becomes trivially easy, as does keeping a moving object in the field of view. Because the mechanical stage provides X-axis and Y-axis verniers, it’s easy to return to a specific location on the slide even after you’ve moved it completely outside the field of view. We wouldn’t even consider using a microscope without a mechanical stage. Life is too short.
Substage Components
Despite the fact that they’re located below the stage (and therefore below the specimen), two substage components have a significant effect on image quality.
Diaphragm
The diaphragm is used to control the diameter of the light cone where it intersects the specimen being viewed. Ideally, you want the diameter of the light cone to be the same size as the field of view of the objective lens you’re using. At low magnification, where the field of view is relatively large, you want a larger light cone; at higher magnification, where the field of view becomes correspondingly smaller, you want a smaller light cone. If the light cone is smaller than the field of view, the field is not completely illuminated. If the light cone is larger than the field of view, “waste” light from outside the field of view reduces contrast and image quality.
Toy microscopes have no diaphragm. Basic models have a disc diaphragm, which is simply a metal disc with several (usually five or six) holes of different diameter that can be rotated into position. Disc diaphragms provide only compromise settings, but are generally quite usable. Better microscopes have iris diaphragms, which can be set continuously to provide any size of aperture, from a pinhole to wide open.
Condenser
The condenser sits between the diaphragm and the stage, focusing light from illuminator onto the specimen to provide a brighter, sharper image. Toy microscopes and entry-level student/hobbyist microscopes have no condenser. Somewhat better microscopes use a simple fixed-focus condenser, usually rated at 0.65 NA (Numerical Aperture, where the NA of the condensor must be at least as high as the NA of the objective lenses it is to be used with. A 0.65 NA condenser can be used with at most a 40X objective. Oil-immersion 100X objectives with a 1.25 NA rating require a 1.25 NA condenser.) Midrange microscopes use a focusable Abbe condenser, usually of 0.65 NA and usually with a spiral focusing arrangement. Better models provide a rack-and-pinion focusable Abbe condenser with a 1.25 NA for use with any objective up to a 100X oil-immersion objective.
Köhler Illumination
If you pick up any book on basic microscopy, you’ll soon encounter the term Köhler illumination. Devised by August Köhler in 1893, this illumination method provides extremely even illumination and the highest possible contrast. Unfortunately, setting up Köhler illumination requires physical features not present on affordable scopes, including a positionable lamp and a focusable lamp condenser. Very few microscopes under $1,000 include the features needed to set up Köhler illumination.Fortunately, the alternative, called critical illumination, is perfectly usable for most visual work. (In fact, many experienced microscopists prefer critical illumination to Köhler illumination for visual work at high magnification.) The extreme evenness of Köhler illumination is important for professional quality results when you shoot images through a microscope, but otherwise critical illumination works fine.
The Final Decision
So, with all of that said, which model should you get? Obviously, that depends on both your needs and your budget, but we can offer some advice to help you make a good decision.
[Editor’s Note: the following products are no longer carried at makershed.com.]
Entry Level 400X microscope:
It’s as easy to spend too little on a microscope as it is to spend too much. We suggest you avoid toy microscopes entirely. They’re a waste of money. If you need a basic 400X scope at minimal cost, choose the Maker Shed Model 109. This scope is perfect for undemanding hobby use or for elementary school students, and in a pinch, can serve through middle school. At $119, it lacks a mechanical stage and provides only basic features, but the optics and mechanicals are solid.
Midrange 400X scope:
If you need a midrange 400X scope, choose the Maker Shed Model 131. This scope is good for hobby use, and can serve a student from middle school or junior high school through high school, excepting AP biology. At $235 this scope provides very good optics and mechanicals. The only major missing feature is the 100X oil-immersion objective, which is needed for cell biology studies in high school AP biology courses.
Entry Level 1000X scope:
If you need an entry-level 1000X scope, choose the Maker Shed Model 134. This scope is excellent for hobby use, and is the only scope a student will need from middle school or junior high school through high school AP biology. At $359, this scope provides very good optics and mechanicals, and is essentially a Model 131 upgraded to include a 100X oil immersion objective, a focusable 1.25 NA Abbe condenser, an iris diaphragm, and a standard mechanical stage.
“Lifetime” 1000X scope:
If you want to make your first microscope purchase your last, choose one of the Maker Shed 160-series models, the $479 Model 160 (monocular), $539 Model 161 (dual-head), $629 Model 162 (binocular), or $819 Model 163 trinocular). You can pay a lot more for a microscope, of course, but the only major feature missing from the 160-series scopes is support for Köhler illumination. Any of the 160-series microscopes is a superb choice for hobby use, and is the only scope a student will need from middle school or junior high school all the way through university and graduate school. The optics and mechanicals are excellent, and the feature list is impressive. Even people who use professional-grade microscopes every day are invariably stunned by the level of mechanical and optical quality the 160-series microscopes provide at this price point. The only upgrades we offer on these scopes are the ASC (high contrast) or plan achromatic objectives.
July 21, 2009
