Microscopes have a large price span, ranging from cheap to in-affordable. Especially in an educational setting, money may play an important factor – money which should be well spent, and maybe not even spent at all on unnecessary microscope features.
The following microscope features are probably not needed (or irrelevant) for introductory microscopic investigations in schools. These are the places where one can save money.
- Infinity optics: They are expensive and not yet very common for lower priced educational microscopes. Infinity optics are not interchangeable between manufacturers – to my knowledge there is no single world-wide standard. This can become problematic if individual objectives have to be exchanged, as one is bound to a particular manufacturer. The advantage of infinity optics becomes evident when working with filters or when doing photographic work. For this reason they are commonly employed in research-grade biomedical microscopes. Stay with the 160mm tube length standard objectives.
- 100x oil-immersion objective: This objective requires more experience. The danger is, that students confuse the oil-immersion objective with the regular air-objectives. There is the danger of contaminating these objectives with the immersion oil. Once the specimen slide is covered with oil, it is not possible to use lower magnification objectives anyway, without significant loss of quality. The oil may also stain the sticker of permanent slides. Using oil immersion objectives without oil is also not recommended. The image quality will be very low and students may rotate the objective into the specimen slide while searching for the best focus. If the oil is not properly cleaned, then dust will accumulate over time further reducing image quality. Choose a 100x oil objective if it is an educational goal to teach students to properly use this type of objective or if you need the high magnification (such as for observing chromosomes in cells).
- Phase contrast optics: Generally they are more expensive. These optics will distort the natural color of the specimen as it converts differences in refractive index into difference of brightness. They are useful for investigating unstained bacteria, and it is questionable if this type of investigation is suitable for introductory microscopic courses.
- Köhler illumination: This illumination system is useful when doing photographic work. For students it poses an additional diaphragm to worry about. The system has to be properly adjusted and aligned before use.
- Tension-free objectives: They are used for polarization work. They are expensive and probably they won’t sell them to you anyway if you tell them that you need the microscope for educational or amateur purposes. I’m just mentioning this for the sake of completion.
- Apochromatic objectives: These objectives are corrected for chromatic aberration and are primarily used for photographic work where high image quality is needed. Achromatic objectives are standard for educational work.
- Plan objectives: These objectives deliver an image which is in focus from center to edge. They are commonly used for photomicrographic work. They are expensive and deliver no significant advantages for educational work.