American Society for Clinical Pathology (ASCP) Technologist in Cytogenetics certification (ASCP CG) Practice Test

In a compound light microscope, how does the aperture stop affect resolution and contrast?

1. By increasing the light intensity

2. By decreasing the magnification

3. By changing the angle of the cone of light

4. By altering the focus

The correct answer is: By changing the angle of the cone of light

The correct choice emphasizes the role of the aperture stop in adjusting the angle of the light cone that enters the objective lens of a compound light microscope. This is essential because the resolution and contrast of the image depend on the numerical aperture (NA) of the objective lens, which is directly influenced by the angle of light that hits it. When the aperture stop is positioned to allow more light at a steeper angle, this increases the numerical aperture, enhancing the resolution of the microscope because it enables the objective lens to gather more light from finer details of the specimen. This leads to a clearer and more finely resolved image. Furthermore, adjusting the angle of the light cone also affects contrast. Higher contrast can result from tuning the aperture stop because it alters how much light interacts with the specimen at given angles, which can enhance the differences between structures that might otherwise appear similar under uniform illumination. Consequently, the function of the aperture stop is crucial in optimizing both resolution and contrast by manipulating the light paths entering the microscope, thus improving the overall quality of the observed specimen.