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They are most commonly used in photographic enlarging to ensure that the negative image is accurately focussed on the easel.
Focus finders are designed so that their optical path is exactly equal to the optical path of the uninterrupted light.
In enlarging, this is achieved by mounting an angled front-silvered mirror on a small plinth and using a strong magnifying eyepiece and graticule to examine the reflected virtual image. The enlarger lens is then carefully focussed until the grain structure of the film can be seen in the plane of the graticle.
The device simplifies and streamlines the darkroom printing process, eliminating guesswork when focusing the enlarger. A grain focus finder is especially useful when printing very thin (underexposed) or thick (overexposed) negatives, or ones that lack fine detail to be focused by eye. As it allows you to see and focus on the grain of the film itself, it works just as well with images of all subject matters. A grain focus finder works equally well with all types of enlargers and film types.
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