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Uses of Objectives of a Microscope

Microscope objectives come in many forms with varying uses. Most lab microscopes in use today have several different interchangeable microscope objectives. These objectives magnify an image that has been placed on the stage of the microscope. Typically, there are three to five objectives on a standard microscope that can be unscrewed from the microscope and replaced on more specialized objectives.

Magnification
- The most common type of objective found on microscopes is the standard magnification objective. The normal range on a microscope turret includes, 4X, 10X, 40X and 100X magnification objectives. The eyepieces normally have magnification lenses on them as well. The magnification on the eyepiece is normally 10X; this makes the true magnification 40X, 100X, 400X and 1000X. To use the 1000X setting, the microscope must have very good optics and be stabilized on the bench. While other objectives have differing uses, they all have some level of magnification.
Bright Field Microscopy
- Normally marked BF on the objective, this is the most common type of objective used in microscopy. A sample is placed on the stage and is exposed to a bright light from underneath. The differences in the absorption of the light through the sample can be seen using the microscope. This type of microscopy works well on samples that have contrasting color. Biological samples normally do not absorb light very well and must be stained. This is accomplished by heat fixing a biological sample to a glass slide and placing a drop of colored dye on the sample for a few seconds. The excess is washed away and the color difference can clearly be seen in the microscope.
Dark Field Microscopy
- Dark field microscopy involves changing the light that falls on the sample. A patch is put between the microscope and the light source. The light must move around this patch and only a small, scattered portion of the light is transmitted into the objective. The image that you see appears to have a dark background and the sample appears to be lighted. In this instance, the bright field objectives can be used for this type of microscopy.
Liquid Immersion Microscopy
- In some instances, a special type of objective is called for to increase the resolution. One type, known as an immersion objective, uses liquid to focus the light and provide good resolution at high magnification. At very high magnification levels, the distance between the objective and the sample is very small. This means that the objective must capture as much light as possible to give the viewer a clear image. By placing a small amount of liquid, such as oil, between the lens and the sample, more light enters the lens and provides a clearer image. While most often a special objective is used for this purpose, certain normal objectives with high magnification factors can be used as well.

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