The Fresnel lens which is use in large-format photography is a converging lens in the form of a thin plastic panel with a series of concentric stepped grooves or rings.
Depending on the direction of the basis they will generate enlarging or diminuating effects. The fresnel screen is placed over the groundglass and brightens up the corners of the groundglass image. Their strength of diffusion facilitates evaluation and accurate focusing especially for short focal lengths. Furthermore accurate exposure determination is made possible through the equal distribution of light.