A Fresnel magnifier is a condenser lens incorporated into the center of some ground glass focusing screens to assist with precise focusing. By magnifying the central area of the image, it makes fine details easier to see and allows photographers to achieve sharp, accurate focus.
Fresnel magnifiers are particularly useful in large-format and technical photography, where manual focusing is critical. They help photographers verify critical sharpness, improve composition accuracy, and ensure professional-quality results in images that demand precise detail.