Which type of objective lens causes the center of the microscope field to be in focus while the periphery is blurred?

The type of objective lens that causes the center of the microscope field to be in focus while the periphery is blurred is the achromatic lens. Achromatic lenses are designed to reduce chromatic aberration, which can distort colors and sharpness around the edges. However, they often have limitations in terms of field flatness, which means that while the center of the field is clear, the edges may not maintain the same level of focus. This results in the periphery appearing blurred compared to the clearer central area.

In contrast, plan achromatic and plan apochromatic lenses are designed to provide a flat field of view, ensuring that the entire image (center and periphery) is in focus at the same time. Flat field objectives specifically aim to correct for field curvature, eliminating the blurring around the edges entirely. Thus, the achromatic lens is distinguishable for having this characteristic focus disparity between the center and the periphery.