Microscope Buying Guide
When buying a microscope there are a few things to keep in mind in order to make sure you have a quality instrument that will be useful.
If you are interested in microscopy, then only the possession of a good microscope will satisfy. A good microscope will be easy on your eyes, easy to focus, and a pleasure to use.
What should you look for when considering buying a new microscope? (This covers only high powered compound microscopes)
Optics
Objectives:
"Real" microscope manufacturers build their instruments according to accepted standards. The objectives have standard thread sizes. Almost all their microscope objectives have only the magnification powers of 4X, 10X, 40X and 100X.
With a 10X eyepiece these powers are 40X, 100X, 400X and 1000X. Considering the wavelength of light, only magnification up to 1000X is practical.
To calculate the total magnification, multiply the power of the eyepiece by the power of the objective. All the standard objectives have N.A. which stands for Numerical Aperture. This value is very important since it tells you what resolution the objective is capable of. In other words, how small a detail can be visible. Obviously, the higher power ones have higher N.A. (Poor quality microscopes generally will NOT have N.A.). The N.A. standards for objectives are: 4X = 0.15, 10X = 0.25, 40X = 0.65 and 100X = 1.25.
The N.A. and the power are engraved on each standard objective, like the one above: 10/0.25. The numbers below are the tube length and the cover glass thickness in mm. In the picture above, the tube length is infinity. Most microscopes have 160mm for tube length.
Items important when choosing a microscope:
Standard size objectives.
Objectives with N.A. and power engraved.
Objectives must have standard threads.
The Eyepiece
Most modern new microscopes have wide field eyepieces. They give a pleasing, large field and most of them allow you to wear eyeglasses when using your microscope. A 10X eyepiece is the norm. eyepieces with higher powers give more magnification, but not more resolution, making the image less sharp.
What powers do you need/want?
In my experience, you do not need the high power 100X, oil immersion objective. I removed them from most of my microscopes. They are messy, hard to use, and rarely needed. You can see bacteria using 400X. Of course, if you are a bacteriologist, the 1000X is a must.
I use the 10X objective 90% of the time. This is the ideal power for inspecting pond water, algae, insect parts etc. It is easy to use, has large working distance, and good depth of focus.
If you have extra money, a 20X objective is a great asset. They are great for looking at pond water. They have a reasonably good working distance and more resolution. They are expensive, however. If you get one, get the same brand as you microscope. Others would work, but would not be parfocal, which is annoying when switching between objectives (A parfocal lens is a lens that stays in focus when magnification/focal length is changed.)
Illumination
The illumination of your specimen is very important. You need a beam of light to illumine your subject. This beam, which goes through your subject, must enter the objective with almost the same N.A. as the N.A. of the objective. To accomplish this, there must be a lens, a condenser, between the light source and the subject you are examining.
Substage or In Stage Condenser
There are many very good standard size microscopes on the market which go up to only 400X. These are good bargains and probably more than what most people would ever need. One thing to watch out for, does the microscope have a condenser.
The condenser can be below the stage or built into the stage. If you ever use 400X (and you will), a condenser is a necessity. This condenser also must have an N.A., and it should not be less than 0.65. These 0.65 condensers are usually built into the stage.
There is also a rotating disk under this type of condenser with various size holes to adjust to the N.A. of the objective. It is not there to regulate the amount of light. Remember, resolution is directly proportional to the N.A. The light coming from below the stage must have the same or slightly less N.A. as the objective. This assures good resolution and contrast. Making the opening smaller will increase contrast, but decrease resolution.
If you do want a microscope which has the 100X, oil immersion objective, then you need a substage condenser with an N.A. of at least 0.9. These are mounted below the stage and moveable. Instead of the rotating disk, there is an adjustable diaphragm to adjust for the light beam's N.A
The light source
Most new microscopes have built in light sources, which for the most part are adequate. Some microscopes are advertized as having Koehler illumination. Real Koehler will only be found on microscopes costing thousands of dollars.
If the microscope does not have some of the illumination system sticking out the back, its not real Koehler. Koehler illumination requires a long light path. You do not need a fancy light source. I have used a frosted 10 watt light bulb on a microscope which had only a mirror and achieved great resolution and pleasant images.
The Stand
The stand must be metal, sturdy and solid. Under high magnification, any wobble is also magnified and very annoying. The classic horseshoe based stand is still sold and a good value for a standard sized microscope, but using one of these for a while can be hard on your neck. These stands can be tilted, but if you are examining little beasties in a water slide, tilting is not a good idea. Modern microscopes have an inclined tube, making longer use more pleasurable.
Focus
If you want to use 400X or higher, then you need a fine focus control in addition to the course focus. Also, a mechanical stage will be necessary to move the slide. At this high power it is very hard to do move the slide without one. A lot of better microscopes have them built in. If they are not built in, they are available for less than $30 on eBay.
Where to Buy a Microscope
The internet is an excellent source. While there are many, I do not want to recommend any particular dealer.
What I do when I buy an item from the internet, I go to Amazon.com and look up the item. Amazon seems to have a more honest rating system. I do check out the lowest rating. Most ratings have to be examined carefully. I have seen a guy berate a good microscope, but what he was upset about was the software that came with the included camera. I have also seen 5 star ratings on junk microscopes. I wonder who wrote this one? Be alert!
Used Microscopes
This is a very tricky subject. You can get amazing deals and also get a boat anchor. The main thing to watch out for is what I pointed out for buying new microscopes. Also, ask many questions. I bought some very good microscopes on eBay. I also bought some junk on purpose with the intent to fix them up and I had a lot of fun doing it. The resulting instrument turned out well, but for the money I paid, I could have had a good, new microscope, but where is the fun in that?
What not to buy
Never buy any cheap, plastic microscopes, usually sold as kits with lots of paraphernalia to make it look like it is scientific lab equipment. Some of those sell even under famous trustworthy names. If you buy online, how can you know what kind of microscope it is? Well, the price should be your first warning. You cannot get a new good microscope for under $60. I bought one for testing and it is utterly useless and frustrating to use.
Another clue is the magnification claimed. If they are not 40X, 100X, 400X, then you can be sure that the microscope is not a standard microscope, but a toy. Some of those toys claim magnification up to 1200X. To sell this junk should be a crime. How many kids gave up on a wonderful hobby because of this garbage?
Conclusion
The plastic toys, which claim quality and great magnification, do not have standard size objectives. Their objectives are not achromats. That is, they are not corrected for chromatic and spherical aberration. To get around this, they use only a small portion of the lens, which cuts down on any N.A. they might have had. Besides their lenses are made of plastic.
If you buy a standard size microscope with standard size objectives, even a cheap one, it will most likely make you happy with your purchase. I see binocular standard size microscopes with all the bells and whistles, selling for less than $200. Judging by the reviews I read, they seem OK, although not the quality of a similar instrument, which would sell for $800 or more. I can almost guarantee that these more expensive microscopes won’t show you more than the cheaper ones will, as long as they are standard models.
I have a few microscopes where I paid over $1000 and I have a cheap one, which goes only up to 400X. Up to this power, it compares well with my more expensive microscopes. Get a standard size instrument and the chances are that you will get a well performing microscope.