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

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What level of openness in the aperture diaphragm allows for the best resolution?

Half open
Completely open
Partially closed
Two-thirds open

The correct answer is: Completely open

The best resolution in microscopy is achieved when the aperture diaphragm is completely open. An open diaphragm allows for maximum light to enter the optical system, which increases the numerical aperture of the lens. This enhancement of light collection improves image clarity and detail, facilitating the visualization of fine structures within the specimen. Additionally, a fully open diaphragm minimizes the effects of diffraction that can occur with tighter apertures. When less light is let in, such as in partially closed options, there is a risk of losing resolution because the contrast and brightness of the image can decrease. The aperture setting directly impacts the depth of field and the overall image quality; hence, operating with a completely open diaphragm tends to optimize the microscope's performance in terms of resolution, making it the preferred choice for detailed observations in cytogenetics and other microscopic analyses.