Fresnel Screen M 679 / Techno
For increasing the brightness of the image corners when using short focal lenghts so called Fresnel screens are used. With the Linhof M 679 and Techno the optical centre of the Fresnel screen is offset from the centre of the screen. In addition it can be moved up and down which means that the brightest part can be positioned on the darker portions of the focusing screen. Because of this a right angle viewfinder can be used without image vignetting. By turning the complete groundglass back by 90° also the darker parts of the lateral shift can be compensated.
The Fresnel screen is placed over the groundglass with the grooved side towards the lens. It is slotted into the 2nd groove of the groundglass back permitting easy moving up and down.