How to choose a stereomicroscope

The entomological survey for the purpose of research, or simply the exploration of the incredible shapes hiding in the small world, requires the use of a stereoscopic microscope – also called stereomicroscope or stereo microscope.

In summary, a stereo microscope is a high-magnification lens through which you can observe three-dimensional subjects. It is a rather simple optical-mechanical system: a so-called head with two observation tubes, an inner prism and lenses system and an apical lens (or objective). The knob on the column lets you move the head in order to manually focus the subject. The result is a three-dimensional view of the subject at magnifications typically ranging between 5X and 200X.

Here it is the drawing of a Olympus SZ Series trinocular stereomicroscope:

Features to look for

The features that you need to look for in a stereo microscope start obviously from the system’s optical quality, which should ensure high definition, contrast and clarity of the image from the center to the edges of the field of view. The field of view should be as broad as possible and wide-field eyepieces – marked with the “WF” acronym (Wide Field) – are a must.

The focusing distance should be good enough to let you work with the fingers under the objective. Classic microscopes like the Olympus SZ40 – or its many clones – ensure about 100mm of focusing distance, which is optimal. Depending on the case, this distance can be varied – increased or decreased – through the use of additional lenses.

The magnification of stereo microscope may be in steps, fixed, or continuously variable (zoom). For routine investigations, in which you need to make comparisons or measurements in series, it is preferable to have fixed magnifications (eg. 10X, 20X, 40X). Some microscopes have graduated zoom knobs with click-stops at the main steps of magnification.

Standard eyepieces have a magnification of 10X, but they can be interchanged with 15X, 20X, 25X or 30X eyepieces, which increase the total magnification without varying the working distance. The total magnification of the microscope is the product of the magnification of the eyepiece and that of the objective. For example, with a 10X eyepieces and a 1X objective you will get a total magnification of 10X (10 x 1 = 10); with a 20X eyepiece the total magnification becomes 20X.

But also the magnification of the objective can be varied thanks to the aforementioned auxiliary lenses, usually with 0.3X, 0.5X, 1.5X, 2.0X magnifications. These auxiliary lenses vary the total magnification and vary the working distance. For example, with a 10X eyepiece, a 1X objective and a 2X auxiliary lens you get 20X total magnification. The same would be obtained with 20X eyepieces and a 1X objective. But positive-power auxiliary lenses (1.5X, 2X) decrease the original focusing distance, while negative-power lenses (0.3X, 0.5X) increase the working distance, particularly useful when you cannot get too close to the subject under the microscope – or vice versa.

Magnification chart for Olympus SZ – and clones – stereomicroscopes

The eyepieces tubes of the modern microscopes bear the knobs for the diopter correction – usually ± 5 diopters – allowing the focused vision also to eyeglass wearers, when they don’t wear them. In any case, the eyepieces born from a good optical design allow a good view also to eyeglass wearers with goggles worn, thanks to their high exit pupil. In addition, the interpupillary distance must be adjustable to adapt to our personal distance, to avoid the vision of a black field.

Lighting is a key issue. The ideal lighting is a cold-light ring – or close to daylight, about 5500°K. Today there are many affordable models of Fluorescent or LED ring illuminators, adjustable in intensity (dimmable) and capable of illuminating without creating shadows.

The photographic opportunities through the microscope are considerable. You can choose between trinocular microscopes – in which a third photo/video tube is connected to an analog/digital camera or to a USB camcorder – or you can find specific adapters for the standard binocular tubes. You can ask for such adapters to the online shops, directly to manufacturers or to eBay ads. And there are already several forums where you can post messages and perceive the secrets of the photography through a microscope – the photomicrography.

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