I purchased a microscope earlier this year, and, as several people have recently asked questions regarding what to look for in a scope, I thought I'd list some of the things that I considered.
Obviously, price is a prime factor, but the following items are all worth noting when establishing whether the price/specification of your potential purchase will be suitable for your present/future needs:- Monocular:-
Has only one eyepiece – should be the least expensive.
Less comfortable for prolonged use. Binocular:-
Much more comfortable for prolonged use.
Must have “intra-ocular” adjustment, so that you can align the eyepiece lenses with your eyes.
At least one of the eyepiece tubes must have dioptric adjustment. Trinocular:-
Basically the same as a binocular, but with an extra port which will accept photographic equipment.
When considering photomicrography, having a trinocular head avoids the need to use one of the eyepiece tubes for camera attachment. Lighting:-
Look for a stand with inbuilt variable intensity lighting. - Modern day microscopes will typically have 20 or 30 watt fully dimmable halogen lamp.
Look for a stand which can be set up for Kohler illumination.
Kohler is a method of critical illumination, whereby the best possible resolution (for the particular scope) of the viewed image is obtained.
Better specced models may have a separate lamp housing condenser iris, which is of benefit when setting up for Kohler illumination. (NB: This is not to be confused with the field condenser iris). The Sub-Stage Condenser:-
Look for a model with a good quality Abbe “Sub-Stage Condenser”. This will have an adjustable field iris which needs to be adjusted to correctly set up the lighting for each objective lens.
Depending on the model, the condenser may or may not have a swing out “bullseye” lens (usually only used with objectives of less than x10), and it may also have one or more swing out filter holders. Turret/Revolving Nosepiece:-
The part that accepts the objective lenses. – Look for one which will take at least four objective lenses. Objective Lenses:-
Most modern day microscopes use infinity corrected lenses.
There are technical explanations to be found, but suffice to say, this invariably means that you will only be able to fit the manufacturers own lenses to the scope.
If buying second hand, you may be able obtain a scope which takes non-infinity corrected lenses, which (in theory) allows interchange between lenses/stands of differing manufacture.
Look for x10, x20, x40, x100 as being the basic requirement.
(The x100 will be an oil immersion lens).
Achromatic lenses are least expensive – usually quite adequate, but are optically inferior to:-
Plan-Achromatic lenses. – Plan Achro’s should fulfil all normal requirements.
Apochromatic lenses. – Top of the range, with prices to match, and generally only available on manufacturers more expensive range of microscopes. Eyepiece lenses:-
Generally much less expensive than objectives.
Look for paired lenses if buying a binocular stand.
Widefield eyepiece lenses offer a generally wider, and slightly flatter field of view.
X10 eyepieces are the standard.
(Don’t be tempted to splash out on x15 eyepieces unless you really feel they will be of some benefit. - They won’t offer any better resolution as they are only magnifying to a greater extent the image produced by the objective lens). Measurement:-
Inevitably, the need to accurately measure spores etc. will arise.
To do this will require an eyepiece graticule, which is a very fine measuring scale etched onto glass.
Separate graticules are sometimes available – which you would then need to ensure would correctly fit (inside) your particular eyepiece lens.
But these days, it is usual practice to buy a measuring eyepiece, - which will already have the graticule fitted, will be of the same magnification as, and will be matched to the normal eyepieces for the particular scope.
In order to calibrate the eyepiece graticule, you will also need a stage graticule.
This is a special microscope slide with a very accurate scale etched onto glass.
Good ones are quite expensive items, and are only needed once, when calibrating the eyepiece graticule for each objective. (If you can borrow one – possibly from your group or a group member if you belong to a local fungus group, so much the better). Focussing:-
Look for a stand with both course and fine focus controls.
Fine focus control is essential at the higher magnifications.
Some scopes have these controls mounted separately, and some have them as two concentric dials. The Stage:-
The part upon which the microscope slide sits.
Look for a “mechanical” stage, i.e. one that has X-Y controls, so that you can effortlessly and accurately move the slide left-right / forward-backwards around the stage, without taking your eye from the subject. For the future:-
Does the model that you are considering come from a well known / reputable / long established supplier?
Are spares available? – (replacement lamps / eyepieces / objective lenses / photographic adapters / graticules, etc. etc.). A few useful links:- Using a Microscope (MushroomExpert.Com) Under the microscope - Scottish Fungi Microscopy-UK full menu of microscopy and microscopes on the web