Focusing and Finding Specimens with a Microscope

Using Parfocal of a Compound Light Scope to View Microscopic Objects

Sep 15, 2008
Tami Port

Objective Lenses of Microscope - Tami Port

Students new to using a microscope often get frustrated when trying to view objects, particularly at higher magnifications. Here's how to do it sans the aggravation.

The magnification of a compound microscope is the result of two sets of lenses:

the objective lenses
the ocular lens or lenses

The ocular lens is closest to the eye, and usually magnifies objects ten times (10X). The objectives are a collection of additional lenses on the rotary nosepiece.

Most compound microscopes have three or four objective lenses. If three, they usually include:

scanning objective, magnifies objects 4X actual size
low power objective, magnifies objects 10X actual size
high dry objective lens, magnifies objects 40X actual size

Finding and Focusing on Microscopic Objects

Start with Clean Lenses

It is important that microscope lenses be very clean. The larger objective lenses (high-dry and oil-immersion) typically tend to get dirty, since they are long enough to sometimes touch the sample. Before trying to view through a scope, use (only) lens paper to gently clean the lenses.

Begin at Low Power Magnification

Even if viewing a very small object, such as a cell, always begin by viewing the object through a low power lens. Depending on how small the object is, start with the scanning or low-power objective.

Using low-power, get the target object centered in the field-of-view (the area that you are seeing through the scope) and in focus as crisply as possible, first by using the coarse focus (usually the big focus knob) and then fine-tuning the clarity of the image with the fine focus (usually the small focus knob, sometimes located within the larger coarse focus knob).

Once the object is in focus, switch to the next higher objective power. Make sure that, when turning the nosepiece to change objectives, you can feel and hear the objective snapping into place. Do not change the focus or manipulate the focus knobs in any way while changing objectives, since doing so will take the object out of focus.

The Importance of Parfocal

A set of objectives on a microscope are said to be parfocal if the viewer can change from one to another and still have the specimen nearly in focus. This is a very convenient feature, because as the magnification increases, even small manipulations of the focus knob can take a specimen far out of focus.

After changing to a higher objective (such as high-dry or oil-immersion) the viewer needs only manipulate the fine focus knob. Never manipulate the coarse focus at high power. In addition to making it very difficult to find the object that had previously been in focus, manipulating the coarse focus at high power can smash the lens into the slide, potentially damaging the scope and the specimen.

When In Doubt, Back It Out

Students commonly have difficulty finding objects under high magnification. If the object seems to disappear after switching to a higher powered objective, switch back to the next lowest objective and try again. This is one of the most important bits of microscopy advice for those new to using a microscope. Students often waste a lot of time searching for an object at higher magnifications, when just flipping back to a lower power, recentering, refocusing and then trying again at a higher power is all it takes to easily find the object.

Viewing Objects under Oil Immersion

To view bacteria under oil immersion, a bacterial smear must first be stained and then there is a series of steps that must be followed in order to get good results. For instructions on how to do this, see the article Viewing Bacteria under Oil Immersion.