The good news is, that there are not many possibilities here. Consider the following:
- 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. /li>
- Optics standards: In recent years large microscope manufacturers have migrated towards so called infinity-corrected objectives. Be aware that these objectives are not compatible with the finite 160mm tube-length standard that 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 purposes. In any case, do not combine objectives of different manufacturers or infinity-corrected optics with a microscope using a finite-optics standard. Many cheaper microscopes still adhere to the finite-optics standard and this is still commonly found in educational microscopes.
- The objective magnifications: beginning microscopists should get 4x, 10x and 40x objectives. A 100x oil immersion objective is optional, and useful for specific applications. It requires more experience, and is more expensive. The question is, if it is worth it. This depends on one’s observation interests.
- The body: it should be quite heave for stability reasons and be made of metal.:
- illumination system: You have a choice between LED, halogen and tungsten. Halogen and LEDs are preferred, there is less heat development. Research grade microscopes still have mostly halogen lamps installed, this is expected to change as LEDs increase their brightness. For photography and high power observations you need more light. The minimum recommended power (halogen) is about 20W. Avoid the purchase of instruments that 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.
- The 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 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.
- The 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.
- The condenser: Amateurs will generally use a bright field Abbe condenser with an integrated iris diaphragm. Cheap 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. This one is needed for dark field observation.
- Köhler illumination: This is recommended if you intend to do photographic work. It ensures even illumination and reduces stray light, which would reduce contrast.
Ask yourself these questions:
- Aspects that speak for 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.
Regardless of specific interest, the microscope should have a condenser (with diaphragm and filter holder), a mechanical stage (and not stage clips), a lamp (and not a mirror, unless the scope should be used in the field), brightfield achromatic objective (and not expensive apochromats, and or phase contrast objectives, unless you delibarately want them).