Stops

Both types of microscope may have two adjusters on the light input. One is the aperture stop, which is usually marked by the letter A. The other, rarer stop, is the field stop, usually marked by the letter F. These are both used to restrict the amount of light getting to the specimen, but have different effects.

The condenser is a collimated light source which produces parallel rays of light. The aperture stop is close to the condenser and it reduces the numerical aperture of the lens. As such it has the effect of reducing the resolution, increasing the depth of field and may increase the contrast. However, if the stop is closed to a very small hole, it has the effect of producing diffraction fringes, which can be misleading. Generally it is bad practice to operate the microscope with such a small aperture

The field stop restricts the size of the circle of light that reaches the specimen surface. This is used to reduce flare and unwanted reflections within the microscope.

As a general rule both of these stops should be kept nearly completely open, and it is rare that adjustment of these will be needed for standard observations.

Use of scale bars

The easiest way of measuring the size of a feature under a microscope is to relate it to the size of the field of view. The simplest way of achieving this is to measure the size of the field of view at a low magnification, and then scale the size appropriately as the magnification is increased. The field of view can be measured approximately by looking at a ruler under the lowest magnification lens.

Accuracy can be improved by using a graticule. A graticule is a slide with a very fine grating which, if metric, will usually measure 1mm across, and is divided into 100 segments, i.e. each segment is 10 µm across. This allows much greater accuracy in measuring the field of view, and so greater accuracy in measuring features.

Metric graticule in polarised light

On some microscopes, a scale bar is superimposed on one of the eyepieces, which can be used to further improve the accuracy of measuring feature sizes. The scale bar can be calibrated by observing either a graticule or a ruler at a low magnification. For example, if 1 division is equivalent to 20 µm with a x5 magnification lens, each division is equivalent to 2 µm with a x50 magnification lens. By measuring a feature using the scale in the eyepiece, the actual size of the feature can be calculated by knowing the width of the divisions in the eyepiece. The scale bar on the eyepiece is particularly useful because it can be rotated and so both widths and lengths can be measured without rotating the specimen.