The types of microscopes a laboratory chooses between: what brightfield, phase contrast microscopes, a dark field microscope, fluorescence, confocal microscopes, stereo and electron instruments each resolve, how the microscope types on a laboratory microscope bench differ once laboratory microscopes are compared side by side, and how to pick from the question rather than from the specification sheet

The types, and the question each answers

1. Brightfield, for stained sections and anything with absorption contrast. The simplest arrangement and the right one for histology, blood films and stained preparations. It is nearly useless for unstained living cells, which have almost no absorption contrast, and that limitation is what every other technique below exists to address.
2. Phase contrast, for living cells in a culture vessel. Converts the phase shift a transparent cell causes into visible intensity, working through plastic vessels. It is the default for tissue culture observation and it needs a matched condenser annulus and phase objective rather than being a setting.
3. Fluorescence and confocal, for specific molecules. Fluorescence localises labelled molecules; a confocal adds a pinhole that rejects out of focus light, which is what makes optical sectioning of a thick specimen possible. If the specimen is thin and flat, widefield is faster, cheaper and often sufficient.
4. Stereo, for anything you need to manipulate. Two optical paths give a three dimensional view at low magnification with a long working distance, which is what dissection, sorting and assembly under the instrument actually require. It is a different job from a compound microscope, not a smaller version of one.
5. Electron and scanning probe, below the diffraction limit. Light microscopy stops at around two hundred nanometres. Electron microscopy goes far below that in vacuum on prepared specimens, and atomic force microscopy gives true height data in air or liquid at lower lateral resolution. Both are specialist instruments best accessed through a facility.

Upright against inverted, which is a sample question

An upright stand looks down through a coverslip and suits slides. An inverted stand looks up through a vessel base and suits anything in a dish or a flask with medium above it. Choosing wrongly here cannot be corrected with objectives.

For a laboratory doing both, two stands is usually the honest answer. Converting one to do the other's job produces an instrument that does neither well.

Where the money should go

Objectives, then the illumination, then the camera, then the stand. A good objective on a modest stand outperforms the reverse, and the most common purchasing mistake is a well specified body carrying the cheapest lenses in the catalogue.

For fluorescence, the filter sets and the light source matter as much as the objective. Mismatched filters are a frequent cause of a new fluorescence instrument disappointing its buyer.

What a demonstration should include

Your own samples, in your own vessels, with the objectives you intend to buy. A demonstration on the vendor's prepared slide proves the instrument works and tells you nothing about your specimen.

Ask about service, objective cleaning and what a replacement lens costs. Objectives are the expensive consumable of a microscope's life and their price is rarely part of the comparison.

Common questions

Which of the types of microscopes do I need for cell culture?

An inverted phase contrast instrument. Cells grow on the vessel bottom with medium above, so the optics must look up through the plastic, and unstained cells need phase contrast to be visible at all.

Do I need a confocal or will widefield fluorescence do?

Widefield for thin, flat specimens such as cultured cells on a coverslip, where it is faster and gentler. Confocal when the specimen is thick and out of focus light is drowning the plane you care about.

What is the practical resolution limit of a light microscope?

Around two hundred nanometres for conventional techniques, set by diffraction. Super-resolution methods go below it with specific labelling and acquisition requirements, and electron microscopy goes far below it on prepared specimens.

Should a laboratory buy a microscope or use a facility?

Buy the routine instrument people use daily, which is usually a phase contrast inverted stand. Use a facility for confocal, super-resolution and electron microscopy, where the instrument is expensive, intermittently used and much better with an expert beside it.

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