If you want to buy a microscope for amateur microscopy, then I recommend the following features.
The good news is, that there are not many possibilities here. Most microscopes these days share the same common features. Introductory microscopes (which cost $/€ 100 or less) are missing some of these.
The microscope body
The body of the microscope houses the lamp and carries the stage, objectives and tube for the eyepieces. It should be quite heave for stability reasons and be made of metal. Every vibration (bumping against the table, walking in the room etc) is magnified by the microscope. A heavy microscope reduces these vibrations.
Objective magnifications
Pretty much all microscopes have a 4x, 10x and 40x objectives. Microscopes of the $/€ 200-300 range and above often also come with a 100x oil immersion objective. This is used for high magnifications of up to 1000x and is the part that drives up the price of a microscope. The question is, if it is worth it. This depends on one’s observation interests. I generally think that the 100x oil objective should be exchanged either with a 20x objective (which is very good for observing water life) or a 60x objective, becasue they can be used more often. Many microscope companies prefer to include a 100x oil immersion objective because they can then sell the microscope with a higher magnification and this makes it apparently more attractive for the consumer. I think that other objective combinations would be more useful. Sometimes the companies do not sell separate 20x or 60x objectives and then I recommend that you try to find it on Aliexpress, becasue most objectives are made in China.
The condenser
Amateurs will generally use a bright field Abbe condenser with an integrated iris diaphragm. Introductory microscopes and toy microscopes do not have one. The condenser bundles the light, increases resolution and is responsible for a brighter image. The condenser also has a filter holder, often a swing-out type. I recommend microscopes with a condenser because the filter holder makes many other observation techniques possible (Darkfield, Rheinberg, Oblique). A condenser is especially useful for the higher magnifications (40x and more) as it improves image quality significantly. Children microscopes often do not have one, not only to keep the cost an size down, but also becasue it simplifies the handling of the microscope.
Spring loaded objectives
Especially at high magnifications the working distance between the specimen and the objective can be the fraction of a millimeter. One careless rotation of the focus knob and it is possible to smash the objective into the specimen. This may result in the destruction of both the specimen (cheap) and the objective (expensive). In order to avoid such damage, manufacturers have introduced spring-loaded objectives. The lower part of the objective is flexibly installed and pushed in when contacting the specimen slide. Chances are pretty good that the higher magnification objectives are spring loaded anyway.
Optics standards
In recent years large high-end microscope manufacturers have migrated towards so called infinity-corrected objectives. Most mid-range microscopes (which are common among amatuer microscopists) use the traditional 160mm standard. Be aware that infinity objectives and 160mmobjectives are not compatible. The 160 standard was introduced in the 19th century and has remained popular up to date. The infinity optics offer several advantages, many of which are probably not relevant for educational or amateur microscopy purposes. In any case, do not combine objectives of different manufacturers or infinity-corrected optics with a microscope using a finite-optics standard. You know which standard you are using by looking at the engraving on the objectives.
Illumination system
You have a choice between LED, halogen and tungsten. Halogen and LEDs are preferred, there is less heat development. Most modern microscopes these days come with LED. Some research microscopes still have halogen lamps installed, because doctors were trained to see the colors using Halogen light. This is expected to change as the LED technology improves. For photography and high power observations you need more light. Avoid the purchase of instruments that only rely on natural lighting and a mirror, unless you need a field microscope. Artificial lighting makes the device independent of natural lighting and can therefore also be used in the evening time. If your microscope has a mirror, then you can use a desk-lamp as a light source.
Microscope head
Here you have three choices. Monocular heads are designed to be used with one eye. Binocular heads for double eye viewing. In compound microscopes you still do not obtain a stereoscopic view, however. Trinocular heads have a third vertical tube, a phototube, for attaching a camera. While it is possible to attach cameras to the eyepieces, it is much more comfortable to use the dedicated third tube for photography.
Stage
This part holds the specimen. A mechanical stage, which allows for the moving of the specimen in two directions, is recommended over simple stage clips for holding the slide. Moving of the slide by hand exerts pressure on the stage and this can result in a loss of focus. A mechanical stage allows the movement of the slide with two rotating knobs. Mechanical stages should be equipped with a scale that simplifies the finding of relevant specimen parts. The slides can then be labeled with the coordinates. Clips are useful for smaller microscopes used in fieldwork and for introductory microscopes.
Köhler illumination
This is recommended if you intend to do photographic work. It ensures even illumination and reduces stray light, which would reduce contrast. This is mostly found in more advanced microscopes.
Additional considerations
Before buying a microscope, I recommend that you also consider these aspects:
- Portability: monocular head (single eyepiece), small body. These may not be as extensible.
- For field work: small body, battery operated or mirror, or stereo microscope
- For viewing for a long time: Binocular head, wide field eyepieces.
- For photography: Trinocular head, Köhler illumination, eyepiece USB camera mounted on the trinocular head. it is then possible to take pictures and visually observe at the same time.