Basic mycology does not require an expensive or fancy microscope: a modest brightfield compound microscope is ideal as long as it can provide magnifications to about x1000, and the optics and mechanics are of reasonable quality.
The cheapest way to get a decent microscope is to buy used, and at the time of writing a serviceable ex-laboratory microscope can be bought for between £100 and £200 from a reputable dealer of used instruments. It is also possible to buy from ebay and private sellers, but be careful as the optics and mechanics are easily damaged by a careless owner (or by someone who does not know how to pack an item for shipping).
The alternative is to buy a new microscope, and an example from a big name manufacturer can easily cost upwards of £2000. Cheaper microscopes are of variable quality, and anyone contemplating a purchase would do well to seek advice and carry out plenty of research beforehand. Online forums can be a good source of information.
A typical compound microscope is supplied with three objectives - x10, x40 and x100 - and one or two x10 eyepieces, providing magnifications of x100, x400 and x1000.
Low and medium power objectives are usually dry objectives, whereas the highest power objective - normally x100 - will be an oil immersion objective, requiring a small quantity of immersion oil to be placed between the objective and the cover slip.
The standard objectives supplied with microscopes are achromatic which means that they are corrected for spherical and chromatic aberrations at two wavelengths. Semi-apochromatic or apochromatic objectives are more highly corrected for specialist applications, but they are significantly more expensive, and are certainly not needed for basic mycology. Note that the maximum theoretical resolving power of an objective is indicated by the numerical aperture (N.A.).
It is worth having Plan objectives which allow the centre of the field and the field edges to be brought in to focus at the same time.
The field of view of the eyepiece is indicated by the field number (FN) which is defined as the width in millimetres of the field seen by the eyepiece. A value of at least 20mm is desirable.
For measuring spores, one of the eyepieces must be fitted with a graticule, which is a glass disk with a ruler engraved onto one surface. The microscope must also have a mechanical stage allowing accurate positioning of the specimen.
Most modern microscopes have a built-in light source, usually a quartz-halogen bulb, though LED light sources are also available. Older instruments might have a simple mirror and require an external light source. In my opinion such instruments are best avoided.
Just above the light source there is usually a field iris, which may be stopped down to reduce stray light from the light source.
The light from the source is focussed on to the specimen by means of a lens (or lenses) known as a condenser. It is important to make sure that the condenser is set to the correct height and that it is centred.
Just below the condenser sits the condenser iris, which acts exactly like the aperture in a camera lens. Reducing the aperture of the condenser iris reduces the effective aperture of the objective, thereby increasing contrast at the expense of resolution and brightness.
The condenser and the two irises should be set up for Koehler illumination to ensure optimal resolution and contrast as described here.
Modern brightfield microscopes use one of two optical arrangements, namely finite tube length optics, which is the older design, and infinity optics which is a new design that appeared in the late twentieth century, and which has been adopted by all of the big names.
In the finite tube length design of microscope an objective forms an image which is magnified by the eyepiece. The distance between the objective and the eyepiece is a fixed value, known as the tube length, and it is usually 160mm or 170mm. This design has the advantage that there are large numbers of compatible objectives and eyepieces on the used market at very reasonable prices.
In the infinity optics design of microscope, the objective produces a parallel bundle of light rays. These travel along the microscope tube to a small convex lens known as a tube lens, where they are focussed to create a real image, which is then magnified by the eyepiece. The key advantage of infinity optics is that accessories such as polarisers can be introduced in to the light path between the objective and the tube lens with a minimal effect on image quality. Unfortunately for the amateur microscopist, infinity optics from one manufacturer cannot be used with a microscope from another manufacturer as each uses a different tube lens specification, and different objective threads. And because infinity optics are relatively recent, there are very few objectives and eyepieces on the used market, and they tend to be rather expensive.
Other types of microscope include darkfield, phase contrast and differential interference contrast (DIC). Such instruments are often very expensive but they allow the study of low contrast structures, which are often hard to observe with more conventional brightfield microscopy.
The big names in the world of microscopes are Zeiss, Leica, Olympus and Nikon. Instruments from these manufacturers are often very expensive, and priced out of the reach of most amateur mycologists.
Another respected name is Meiji, a Japanese manufacturer of stereo and compound microscopes. Their microscopes have excellent mechanics, and decent optics, though they do not offer as wide a range of objectives as the major manufacturers. The instruments are sold at similar prices to the better Chinese instruments. Meiji have recently moved over to the infinity optics design.
Most of the cheaper microscopes are of Chinese origin, although some are also made in India. Although some are best avoided, the quality is getting better, and I am told that the better examples offer excellent performance at budget prices. Even some of the major manufacturers have started to outsource the production of their lower end microscopes to China. The Zeiss Primostar and Olympus CX21 are both made in China.
I am not going to tell you what to buy, as you will have your own needs and budget. But were I to buy a microscope today, I would probably buy a used brightfield microscope with finite tube length optics made by one of the big name manufacturers. These instruments are made to last a lifetime, and objectives and other parts are available relatively cheaply on the used market.
Futher information can be found at the following sites: