There are a range of different features that one should consider when purchasing a new 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.
• Same series: If several microscopes are purchased, they should be of the same brand and make. This allows for an easier exchangeability of parts. Sooner or later different parts will have to be replaced and devices from the same manufacturer keep the costs down.
• 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.
• X/Y Stage: For labwork, give a preference to systems that are equipped with an X/Y Stage and not clips. Moving of the slide by hand exerts pressure on the stage and this can result in a loss of focus. An X/Y stage allows the movement of the slide with two rotating knobs. X/Y stages should be equipped with a scale that simplifies the finding of relevant specimen parts. The slides can then be labeled with the coordinates for students to directly find the part of interest. Clips are useful for smaller microscopes used in fieldwork.
• Light, no mirror: Avoid the purchase of instruments that rely on natural lighting and a mirror. Sooner or later students will aim the mirror directly at the sun. This can cause irreversible eye-damage. Artificial lighting also makes the device independent of natural lighting and can therefore also be used in the evening time.
• Parfocal objectives: Parfocal optics allows for a change in magnification without the necessity of much refocusing. Make sure that the objectives are designed to work with each other in this respect. Parfocality is not automatically guaranteed.
• Focus block: I have seen some systems where even the low power objectives could be crashed into the specimen by careless focusing. I suppose that this is because of different manufacturers of objectives and the microscope. The microscope was, so I suppose, designed to accommodate a wide range of third-party objectives and there was not physical focusing block built in. In this case I recommend that the school purchases microscopes that allows for a manual setting of a focus block.
• Magnifications: I recommend the following objectives: 4x, 10x, 40x and possibly a 100x oil immersion objective. Again, make sure that they are of the same series and designed to work together. A 100x oil immersion objective is useful when cell division events are to be observed, mostly relevant for older students.

Objectives can be classified as follows:

• Parfocal objectives: Parfocal optics allows for a change in magnification without much refocusing. Make sure that the objectives are designed to work with each other in this respect.
• Achromatic objectives: These are the most common and also the cheapest objectives. Chromatic aberration is corrected for two colors. When observing specimens of high contrast it is possible to see red and blue fringes. Achromatic objectives are perfectly sufficient for routine analysis and for educational purposes. They do not, however, possess the resolving power of the better corrected objectives. Some achromatic objectives also display a slight image distortion. Both chromatic aberration and distortion may be annoying when conducting photographic work, but do otherwise not disturb. Achromatic objectives do have other advantages that make them suitable for course work. They have a larger depth of field and the working-distance (the distance between the objective and the specimen) is larger as well. This makes focusing easier and reduces the chance of crashing the objective into the specimen.
• Apochrmatic objectives: These objectives are corrected for three colors. Fringes are not visible and the obtainable resolution is higher. The trade-off is a reduced working distance and smaller depth of field. These factors and a higher price make apochromatic objectives less suitable for course work.
• Plan objectives: These objectives are available for both achromatic and apochromatic versions. They contain additional lens elements that correct the distortions. The cost of these objectives is naturally higher. They are commonly used for photomicrographic work. Especially the planapochomatic objectives deliver images with no recognizable chromatic aberration and distortion.
• Fluorite objectives: Fluorite objections are composed of relatively few lens elements. For this reason the contrast is higher. These objectoves are applied in special areas such as fluorescence microscopy or fine structure research.
• Phase contrast objectives: The phase contrast technique allows for visualization of transparent and uncolored specimens. Unstained bacteria, for example, are very difficult to see using the bright-field technique, but are clearly visible in phase contrast. Phase contrast requires special objectives, however. Phase contrast objectives are available also as achromatic, apochomatic, and plan versions. The microscope itself must also be equipped with an appropriate filter system to use this technique. Phase contrast objectives can also be used for bright field work, but the image quality is lower. Due to the higher cost of phase contrast equipment I recommend that only one or 2 teacher’s microscopes are equipped with this system. These microscopes can then be coupled to a video system for the whole class to see. Before the purchase of the system, the teachers should clearly specify the type of observations that are to be conducted. If much living material is to be investigated – material that can not be easily stained – then phase contrast is preferable. If students are to conduct sample preparation and staining, then bright-field objectives are probably the better option.
• Oil Immersion Objectives: These objectives are commonly used for magnifications around 100x. A drop of immersion oil is placed on the slide and the objective is rotated directly into the oil. Immersion objectives increase the numeric aperture and thus the resolution. They are useful structures inside a cell, such as the chromosomes of dividing cells. In a school setting, oil immersion objectives are a mixed blessing. While they do allow the observation of various sub-cellular structures, significant drawbacks should not be overlooked. It can happen that students confuse the objectives and rotate non-immersion objectives into the oil. If not properly cleaned (a common problem when there is not enough time for clean up at the end of a lesson), then dust will accumulate on the objective lens delivering a blurry image in future session. Students may also attempt to use a high power oil objective without oil. In this case parfocality is not guaranteed anymore and there is the danger that the objecitve is crashed into the specimen. If oil immersion is used, then only synthetic oil should be used. Natural oils may have the tendency to solidify if not cleaned properly.
• Water immersion objectives: These are not commonly used in school educational settings. They increase resolution by immersing the objective into water and not synthetic oil.

It is possible to construct a simple compound microscope using only two lenses: one highly magnifying objective lens and one low magnification ocular (eyepiece) lens. Why then, are modern research and even course microscopes so complex? After all, some modern objectives can contain up to 10 or more individual lens elements.

The simple 2-lens system, although cheap to construct, does have certain drawbacks. The image quality is low and also the color representation is not optimal. Modern microscopes compensate a whole range of limitations and lens errors inherent in simpler systems.

The objective has the most influence in determining the image quality of the microscope. An objective should deliver an image with a high resolution, a high contrast and a low lens error. Naturally it is difficult to achieve all of these goals simultaneously. Several lens elements are necessary to compensate a range of lens errors. This, however, impacts negatively on the image contrast. The reduced contrast must be compensated with an appropriate lens coating. All of these corrective measures naturally increase the cost of the objective.

Numerical Aperture: One of the key values that characterizes the performance of an objective is its numerical aperture. This value is essentially a direct measure of the resolving power of the objective. The higher the numerical aperture, the finer is the visible detail. Objectives with a high numerical aperture are also capable of collecting more light, the image is brighter. Objectives have their numerical aperture engraved on the outside.

Chromatic Abberration: It is in the physical nature of light, that the light waves towards the red end of the spectrum are not refracted as much as the waves towards the blue end. As the white light of the lamp passes through a lens it is split up into different colors. The focal point of the different colors is not the same. This phenomenon is called chromatic aberration. Modern objectives attempt to correct this lens error by coupling of several lens elements. Achromatic objectives are very popular and commonly used in schools. These objectives are optimized to correct two colors of the spectrum. A small amount of chromatic aberration is still visible. The more expensive apochomatic objectives are optimized for three colors. They show no visible chromatic aberration and are frequently employed for photographic documentation.

Spherical aberration: The objectives must also compensate for spherical aberration. Light rays that hit a lens towards the side are more strongly refracted than light rays that hit the lens closer to its optical axis. This effect is also dependent on the wavelength of the light ray. This lens error can also be minimized by a combination of different lens elements.

Field curvature:Cheaper objectives do not produce a flat plane of focus. When the center of the image is in focus, the sides of the image are not in focus, and vice versa. This abberation is due to the fact that lenses have curved surfaces. This is generally not a problem for routine visual observation. It does, however, become very annoying when taking photographs. Flat field objectives correct the field curvature. These objectives are designated with the word „plan“, such as plan achromats, plan apochromats or plan fluorites. [image demonstrating field curvature]. Flat-field objectives, and especially plan apochromats are expensive and an unnecessary luxury for instructional course work.