Fresnel lens which we 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.
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