|"What kind of microscope should I get?"
"What features should I get?"
"What makes a good microscope?"
These are common questions. It's hard to come up with a statement that's true for every buyer, but there is a blanket statement that holds true for almost everyone:
Choose the best features your budget allows.
Ah, but what are the best features?
I'll try to keep this short. If you want a quick summary of what to look for, here goes:
I. The three, most basic features your microscope should have if you're even a little bit serious:
• Fine focus
• Abbe condenser
• Rotating nosepiece with removable 4x, 10x, and 40x objectives, preferably DIN lenses (an international standard, which means you will always be able to get replacement lenses even if your manufacturer discontinues a model).
II. Additional features you should really get if you're serious and your budget can accomodate them:
• Mechanical stage - forget using your fingers to move the slide, especially if you work with high power much. The mechanical stage also allows you to find something repeatedly on a slide instead of playing hit-or-miss each time.
• Oil-immersion objective lens (100x) - allows your microscope to be used at 1000x. Depending on what you're planning to study, this may not be optional. Some cell types and structures simply aren't visible at much below 1000x.
• Binocular head - this is more of an optional feature, but if you'll be using your microscope very often, a binocular microscope feels much easier on the eyes.
Now, let's talk about the basic features in some detail:
1.) Fine focus. You need this if you're remotely serious about microscopy. Powers above 400x absolutely require fine focus; unless the microscope is a toy billing itself as something it's not, any 'scope allowing high-power magnification will probably come with this. The gray area, so to speak, occurs with microscopes that go up to 400x but don't have fine focus. While it is possible to use these, it is not that enjoyable for long-term use.
The now-discontinued Outfitter from LW Scientific, though a great little backpacking scope (and the cutest little thing you ever saw), did not have fine focus. It had only one focus knob: coarse. When it was set up at 400x, you had to hold the knob, or else it would drift out of focus in less than a second (although with practice it was possible to hold the Outfitter's focus quite steady). The lack of fine focus also made it too easy to crush slides with the high power objective.
That's alright for a $67 microscope or even a $125 microscope, but units priced much above that should include both coarse and fine focus adjustment.
2.) Rotating nosepiece with at least 4x, 10x, and 40x objectives. If your eyepiece is 10x (as most are), that gives you a total magnification of 40x, 100x, and 400x.
There is one more objective, the 100x oil-immersion lens; this would give you a total magnifcation of 1000x.
If your budget can't accomodate a microscope with all 4 objectives (4, 10, 40, and 100), try at least to pick a microscope that has a built-in place on the nosepiece to accept the 100x oil-immersion lens. Then you can add that lens to the microscope at your convenience (and when your budget allows).
"Nosepiece" is a strange term. It refers to the microscope's nose-- well, the place one imagines its nose would be, if it had one-- not your nose. The rotating nosepiece allows you to change magnifications while still looking through the eyepiece (which, instead of being located where the microscope's "eye" would be, is located where your eye is... terms that have been around for that long do not always make sense). If you see something on the slide and want a closer look, just rotate a more powerful objective lens into place.
There are a couple of beginner microscopes that don't have a rotating nosepiece, and that's OK for what they are. Sure, you could make some stunning discovery in the remotest reaches of Siberia with a $67 backpack microscope having a fixed lens and no fine focus, but such a microscope is more of a backup unit.
3.) Abbe condenser. Some introductory and student microscopes come with a prism or a mirror to gather light. This helps keep the cost down: you provide the light source. For what these scopes are (i.e., econo models), they're okay.
They may include a multi-aperture disc (which you rotate to adjust brightness) or even an iris diaphragm, but the cheaper microscopes don't offer any way to focus the light beam.
Think of and hourglass or an "X" formed by a focused beam of light. The junction of the "X" (i.e., the point of focus) should be right on the specimen. It requires lenses to achieve this. The Abbe condenser's lenses and the iris diaphragm's adjustable aperture work together to give the right intensity, focus, and ray angle for the magnification being used.
The benefit of a good condenser starts to become most apparent at 400x and above. To get the best image at high power, you have to close down the light aperture. That means less light comes through, making your image appear darker. This presents a bit of a paradox that cheaper scopes cannot manage. (I used to have a toy microscope that advertised "750x" but couldn't even do 300 because the image was so dark.)
Higher-end "research" microscopes also use the Abbe condenser, sometimes with improvements such as an additional iris diaphragm & Koehler illumination. When you need to work at very high power (1000x on up to 1500x, the practical limit for an optical microscope), these improvements become worthwhile.
"Toy microscopes" & "Beginner" units
If the price is above $50 or so, you should expect your microscope to have glass optics. You should also expect reasonably good alignment of the lens system, with higher prices equating to better and more consistent alignment.
Even the $67 Outfitter (when the user could hold the coarse-focus knob steadily enough) afforded a fairly sharp view of the multi-lobed nuclei of white blood cells at 400x. That means the optical alignment was pretty good; the little scope was held back more by its crude focus controls than anything else. For what that unit was, though, it was quite a good deal.
Whenever you see a metallized-plastic wonder that advertises "600x", "700x", or some other very high magnification, realize you're being told what, in advertising lingo, they call a "lie". Even the best glass optics cannot give a good image above roughly 400x to 500x without the use of a special oil-immersion lens; these lenses cost anywhere from $100 on up, more than the cost of an entire "toy" microscope.
Furthermore, the cheap "toy" scopes often don't have good enough alignment to give 400x with any certainty, even when they claim to offer high magnifications. 100x or 200x isn't a problem for these units; it's just too bad that 400x is where you start to be able to see th