This is a 100X objective lens equipped with an adjustable iris diaphragm, making it ideal for dark-field microscopy. Standard bright-field objectives often produce low-contrast images because of their high fixed numerical apertures. In contrast, this lens features a built-in iris that lets you finely adjust the aperture from 0.36 to 1.25. This simple adjustment makes it easy to control and optimize the intensity of dark-field contrast for clearer, more detailed observations.