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

How does closing down the condenser diaphragm by 1/4 to 1/3 affect the observation of chromosomes?

1. It improves resolution

2. It enhances contrast between light and dark bands

3. It increases brightness

4. It reduces field of view

The correct answer is: It enhances contrast between light and dark bands

The response indicates that closing down the condenser diaphragm by 1/4 to 1/3 enhances the contrast between light and dark bands when observing chromosomes. This effect results from the change in how light interacts with the specimen. When the diaphragm is closed, less light is allowed to enter the optical pathway, which improves the overall contrast between the structures being observed. In cytogenetics, where chromosomes are often viewed under a microscope during metaphase spreads, increasing contrast is crucial for clearly distinguishing between the different regions of chromosomes, such as light and dark bands. Light bands consist of less dense chromatin, while dark bands are areas of more condensed chromatin. Enhanced contrast makes it easier to identify structural abnormalities and variations in chromosome morphology. While improving resolution is important, this particular technique primarily affects contrast rather than resolution directly. Brightness may decrease with the diaphragm closed, as more light is being restricted, which can illuminate details less effectively. Similarly, the field of view is not directly reduced; it may appear smaller due to the light limitation, but that's not the main effect being focused on here. Thus, the enhancement of contrast, attributed to the controlled light conditions created by adjusting the diaphragm, directly aids in the effective visualization of chromosomal structures and their arrangements