microscope diagram

How A Microscope Works

In order to buy the best microscope for my needs, I discovered that I needed to know how a microscope works. Without a basic knowledge of microscopes, it’s very difficult to make a good purchasing decision. It’s definitely not just a case of finding the highest advertised magnification and buying that one. If you do that, you will very likely buy a microscope that gives a lousy experience. You will get “buyers remorse” and think that microscopes suck. That would be a shame because microscopes are great!

The information is available online, but it’s spread around different sites, and it’s difficult to put together the essential information needed by a beginner. I could not find even one online article which summarized the essential information of how a microscope works for an amateur.

That’s why I’ve made this post. It’s not an in-depth study of advanced techniques. It’s just the basics for someone who wants to know how to buy a microscope.

The basic information you need to know is very simple. I encourage to you read this whole post through to the end. Then buy a microscope. Seriously, it’s a lot of fun! Fifteen minutes with a microscope will teach you something about how life works. What can you do with fifteen minutes on Facebook?

What Is A Compound Microscope?

A compound microscope is the traditional light microscope that everyone knows. You look through an eyepiece (or binocular eyepieces) at a slide with a specimen on it. It’s greatly magnified and you see all the details, right down to the cellular level.

This post will teach you how the compound microscope parts work together. If you want information on other kinds of microscopes, I’ve written a review of a cheap USB microscope, which is great for cheap macro photography (I’ve got plans for this in summer when there are insects around, so check back for close up images and videos of live insects moving around). But USB microscopes are not much good for looking at or photographing specimens at the cellular level.

If you have $100,000 to spend, you can research electron microscopes and how to put together a laboratory to use one. Of course this post is then way too simple for you to be reading!

Compound Microscope Parts

Compound microscope parts work together very simply:

  • A specimen, such as an insect, blood smear, or bacteria colony is placed on a slide. A cover slip is placed over it.
  • The slide is placed on a microscope’s stage.
  • A light source under the stage shines upwards to illuminate the specimen on the slide.
  • An objective lens is rotated directly over the slide, and magnifies the specimen.
  • One or two eyepieces at the top of a tube magnifies the image projected from the objective for your viewing pleasure.

There are many more details, but this post is to explain how a microscope works to someone with no knowledge of microscopes. I’ll explain each step and the compound microscope parts involved. The explanation will help you choose a good microscope with high magnification and good lighting.

Microscope Slides

Microscope slides
A box of microscope slides and coverslips can be bought cheaply.

Placing a specimen on a microscope slide is a relatively simple procedure. Pop an insect into a drop of mounting medium, or make a blood smear directly on the glass slide, or place a drop of stagnant water teaming with microscopic wildlife on the slide.

Blood smear on microscope slide
A blood smear on a microscope slide
Spider on microscope slide
A spider skin on a microscope slide.
Mold on microscope slide
Some mold on a microscope slide. I grew it in a bit of old yogurt.

You should use microscope cover slips to cover the specimens. This is necessary for oil immersion (which I explain at the end of this post). At 40x magnification, the objective lens is going to come very close to the specimen (even touch the cover slip). The cover slip will protect the objective lens from coming into contact with the specimen and getting damaged.

If you want to create a permanent microscopic slide, you will need to place a microscope cover slip over a specimen in mounting medium.

Microscope Stage

Most modern microscopes have a mechanical stage. A stage is the platform that holds a microscope slide. It has a clip that secures the slide firmly in place. A mechanical stage can move up and down, from side to side, and forwards and backwards. Advanced models can move in a circular direction. For an amateur who just wants to make awesome videos of red blood cells flowing along, and watch microscopic predators hunting their prey, you only need a rectangular mechanical stage in a good quality microscope. Cheap microscopes often have poor quality mechanical controls in their stages. They break easily, so choose a good microscope.

Close to the stage are the focus controls. The coarse focus control is for adjusting the position of the slide relative to the objective. The fine focus control is for finely adjusting the position so that you have a perfect image in perfect focus. Simple.

Microscope Lighting

Microscope lighting
A LED light source in a microscope.

Microscope illumination is created from a light source directly under the stage. There are different types of microscope lighting. For an affordable beginner microscope, the best are halogen or LED lighting. It’s important to get a microscope with a high degree of lighting because at very high magnification light becomes scarce.

The light source is focused through a condenser and into the objective. A microscope condenser is essentially just a lens which focuses the light into a concentrated beam to get more illumination.

Microscope Objectives

Microscope objectives
A close up of the objective lenses in a microscope.

The objective is the main lens of the microscope. A compound microscope has a revolving wheel with 3 or 4 objective lenses attached to it. The objectives will have different magnifications. You can rotate whichever objective you want into place.

The function of objectives in a microscope is to magnify the image of the specimen so you can see the details or even the cells.

Microscope objectives have a broad range with a typical high magnification microscope coming with 4x, 10x, 40x and 100x objectives. You can buy objectives separately to increase the utility of your microscope.

A 100x objective will usually be an oil immersion objective. I’ll explain this in more detail at the end of this post.

Is 2000x Magnification Better Than 1000x Magnification?

No. It won’t increase the detail, but it can be useful to get a blown up image.

The specimen is magnified by the objective. This magnified image is again magnified by the eyepiece. So the total magnification is the objective magnification multiplied by the eyepiece magnification.

A 100x objective multiplied by a 10x eyepiece = 1000x total magnification.

A 100x objective multiplied by a 20x eyepiece = 2000x total magnification.

BUT … the eyepiece magnification does not actually zoom into the specimen. It only magnifies the image projected by the objective lens. The details (cells, fine hairs, etc.) are revealed by the objective lens. The eyepiece just expands the tiny projected image so that you can see it with your eyes.

It’s like taking a photograph with a 2 megapixel camera. Just because you increase the resolution 5x with Paintshop does not mean you now have all the detail of a 10 megapixel photograph. You’ve just got a 2 megapixel photograph that has been blown up. You won’t see any extra detail. In fact, a higher magnification eyepiece may make your image worse because you will only see a small part of the blown up image. It depends on what you are trying to achieve. In most cases, I would prefer to see the whole image. A higher magnification eyepiece is really only useful if you want to make it easier to locate a small bacterium or something by zooming up the image. It won’t show any extra details in the bacterium though.

For example, you use a 100x microscope objective with a 10x eyepiece. You now have a magnification of 1000x. Great magnification! You can see red blood cells clearly with this! So you reckon, “What if I use a 20x eyepiece? I’ll see right into the blood cells and see what they’re made of!” So you swap out the 10x eyepiece for a 20x eyepiece, but you just get a blurry blown up image of just a few blood cells. No increased detail. In fact, the 1000x magnification looked better!

It’s easy to increase your total magnification. You don’t need to buy a microscope advertised as 2000x magnification. The magnification is 2000x because the microscope comes with a 20x eyepiece. You can buy a better quality microscope, and just buy a 20x eyepiece separately. You can get a simple one for less than $15. Personally, I wouldn’t bother. A 10x eyepiece is perfect for seeing the magnified image. In fact, when taking photographs with a dedicated microscope camera, you don’t even use an eyepiece at all. You swap out the eyepiece for the microscope camera.

Types Of Microscope Objectives

A compound microscope has different types of objectives which give different quality results. Most microscope objectives are achromatic or planachromatic. You also get apochromatic and planapochromatic objectives for very clear images.

Achromatic Objectives

An achromatic objective corrects for the loss of color that occurs at high magnification. If not for the achromatic color correction, highly magnified images would look like they’ve only got 2 colors.

Most amateur microscopes come with achromatic objectives. If the microscope doesn’t specify what type of objectives they have, then they’re achromatic. Here are some images of plant cells I took with achromatic objectives:

100x magnification of plant cells under a microscope
Some plant cells under a microscope that have been magnified 100x.
400x magnification of plant cells under a microscope
Plant cells under a microscope that have been magnified 400x times.

Apochromatic Objectives

Apochromatic objectives have even better color correction, but they are expensive and really only of interest to someone who is prepared to spend thousands of dollars to get the absolutely best image possible.

Achromatic vs Apochromatic

Here are two side by side images comparing achromatic vs apochromatic images (scroll a little down to see the comparison). As you can see, the color in the apochromatic image is clearer.

Before you decide to buy apochromatic objectives (which are expensive, and I’m not even sure where to buy such specialized equipment), you’re probably better off buying planachromatic objectives.

Planachromatic Objectives

The lenses of non-plan achromatic objectives compromise between a clear center and a blurry outer edge the image. You can make the center of the image more clear, but the edge becomes blurry, or you can make the edge more clear, but then the center becomes blurry. It’s a balancing act. Don’t worry, it doesn’t look bad as you can see in the earlier images.

Planachromatic objectives correct this distortion with additional lenses to create a more uniform, flatter image. If you’re getting serious about microscopy and you’re looking to spend some money, this is a good place to do so. Unfortunately, I don’t have a planachromatic objective to show you the difference in image quality, and I couldn’t find a good comparison image on the internet either.

Planapochromatic Objectives

Okay, so you’re rich. Then you buy planapochromatic objectives. I don’t know where you buy them. They are pretty much industrial grade objectives for very serious research.

Microscope Eyepiece

Function of microscope eyepiece
A 10x microscope eyepiece inserted into the microscope tube.

The function of the eyepiece in a microscope (or two eyepieces if it’s a binocular microscope) is to increase the size of the magnified image so that you can see it clearly.

Microscope eyepieces come in various magnifications. Typically though, you only need a 10x eyepiece to see everything clearly.

Wide field eyepieces increase the total size of the image you see. They are more comfortable to look through, and you see a bigger image. They are worth the extra money. “WF” means wide field.

20x wide field microscope eyepiece
A 20x microscope eyepiece.

Microscope Camera

If you’re taking microscopic photographs or videos with a microscope camera, you don’t use the eyepiece. You remove the eyepiece and place a microscope eyepiece camera into the tube. The camera has it’s own zoom to magnify the image.

You have a few options when it comes to microscope photography:

  • You can buy adapters for SLR cameras to connect them to a trinocular microscope.
  • You can buy adapters for iPhones and Android phones to hold them over the eyepiece. You can even glue a piece of PVC piping onto a phone cover and use it to hold your phone camera in place.
  • You can just hold a camera over the eyepiece by hand. I’ve done that to take all the microscopic images in this post.
  • You can buy a special camera designed to replace the eyepiece in the eyepiece tube or to fit onto a C-mount on a trinocular microscope.

Binocular Microscope

A binocular compound microscope is a microscope that can take two eyepieces. You look through both eyepieces like a binoculars. If you’re planning on viewing specimens for hours at a time, then a binocular microscope is a good choice. It’s easier to use because your brain does not need to adjust to vision coming from only one of your eyes, and keeping one eye closed for long periods of time is uncomfortable.

Trinocular Microscope

A trinocolar microscope has an extra eyepiece tube which is usually used for a camera. You can also use it for a third eyepiece so someone else can observe a magnified specimen at the same time as you.

It’s nice to have, but I wouldn’t pay too much extra for this feature, since when you take photographs, you typically want to see the digital output on your computer or camera screen anyway. You won’t be looking into the eyepieces at the same time as taking a photograph. However, it makes life marginally easier not having to swap in and out an eyepiece for a camera.

Oil Immersion

If you have a 100x microscope objective, it will most likely be an oil immersion objective (the alternatives are water immersion and silicon immersion). This is because at 100x magnification, the slight refraction of light greatly distorts the magnified image. You need this technique to reduce the distortion.

You will need to place a drop of immersion oil on top of the microscope cover slip.

Oil immersion objectives are used by lowering the objective very close to the slide and adjusting the distance using the fine focus control. The objective will dip into the drop of immersion oil. Now there is a singe medium of immersion oil between the slide and the objective. The immersion oil reduces the refraction of the light as it travels from the specimen through the cover slip through the air and into the objective lens.

Some oil immersion objectives have a correction collar built in. The objective correction collar is an adjustable collar that you can adjust to compensate for the refraction of light passing through the glass cover slip. Think of it as an extra focus control. Get as clear a focus as you can with the fine focus control. Then get a clearer focus using the correction collar.

A warning about oil immersion: Don’t use a non oil immersion objective with immersion oil. You will damage your objective.

How To Buy A Microscope

Now that you know the essentials of how a microscope works, you should definitely buy one. I’ve written a post on how to choose a first microscope that will meet your needs. It will help you even more to make a good purchasing decision.

All the best in your microscopy future!

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