The Micro Focus Finder is a tool to ensure that your image us sharp before exposing the printing paper.
The Paterson Micro Focus Finder produces a highly magnified image of the image being projected onto the enlarger baseboard. This allows the grain structure of the negative to be brought into sharp as the image is adjusted prior to the exposure of the printing paper.
The eyepiece is a adjustable for individual eyesight. This can be done by moving the eyepiece up and down while looking through it until the built in cross hair is sharp. The eyepiece can then be locked into position.