Objective lenses are the primary components for magnifying a specimen. Selecting the correct one to start with is important for effective microscopic observations.
Starting with the Right Lens
The typical starting objective on a compound microscope is the lowest power (4x or 10x), often called a “scanning” or “low power” objective. Beginning with a low-power objective provides the widest field of view, the entire circular area visible through the eyepiece. This makes it much easier to locate the desired area of the specimen on the slide.
A low-power objective also provides a greater working distance between the lens and the slide. This reduces the risk of accidentally crashing the lens into the slide, preventing damage to both. When starting with low power, initial focusing is simpler and less prone to frustration, especially for novice users.
Beginning Your Observation
To begin, rotate the lowest power objective (typically 4x or 10x) into position over the stage. Place your prepared microscope slide onto the stage and secure it using the stage clips.
Adjust the stage height using the coarse focus knob, moving it upward until the objective lens is very close to, but not touching, the slide. While looking through the eyepiece, slowly turn the coarse adjustment knob to move the stage downward until the specimen comes into general view. Once the image is roughly visible, switch to the fine focus knob for precise adjustments to achieve a sharp and clear image.
Proper illumination is also important; adjust the light source intensity and the condenser’s iris diaphragm to optimize contrast and brightness for the best view at this initial magnification.
Moving to Higher Power
After successfully observing and centering your specimen under low power, you can transition to higher magnification objectives, such as 40x or 100x. Most modern microscopes are “parfocal,” meaning a specimen in focus with one objective remains largely in focus when switching to another. This allows for a smooth transition, requiring only minor fine focus adjustments at higher powers.
As magnification increases, the field of view becomes significantly smaller, and the working distance between the lens and the slide decreases. For the highest magnification (typically 100x), immersion oil is necessary for optimal clarity and resolution. This specialized oil directs more light into the objective by reducing light refraction, resulting in a clearer, sharper image of fine details.