A focus finder is a device allowing photographers to check the focus of an image while enlarging it in a darkroom. If an enlarger is not appropriately focused, the resulting image can be muddy and dull. Photographers set up the device when they work with an enlarger to check the focus and tighten it, if necessary, before they move on to test strips and printing of the photograph. Focus finders are widely available and are usually relatively inexpensive.
Photographers enlarging images in a darkroom need to insert film into the holder and position it in an enlarger. Depending on the nature of the image, they can adjust the aperture to determine how much light is allowed through the negative. The enlarger projects a magnified image onto an easel, and the photographer can adjust the degree of magnification to get an image of the desired size. Larger film tends to perform better with big images, as small film can get grainy and blurry as the image size increases.
No matter how the photograph is focused, the enlarger itself needs to be focused with the assistance of a focus finder. The device has a small tube and a mirror that allows the photographer to lean down and see an extreme closeup of what the enlarger is projecting. If the grain of the film is blurred, the enlarger is out of focus, and needs to be adjusted. The photographer can focus the enlarger until the grain pops into sharp focus, and then lock it down to prepare for a test strip to determine the length of exposure necessary to develop the image.
A number of tricks of focus and lighting can be used in the production of images, and a photographer may make a deliberate decision to leave an image out of focus or tweak the focus for a desired aesthetic outcome. If the enlarger is not properly adjusted, however, the resulting photograph can look muddy and blurred. The focus finder allows photographers to focus purely on the grain, not the image itself, to make sure it is tightly, crisply focused.
Digital photographers may also use a focus finder tool in image editing. In this case, the application actually looks for the focal point in the image so the photographer knows where the area of tightest focus should be. A different and also unrelated technology is a focus finder for stage lighting. Such devices communicate with lights in the rigging to allow them to focus on a specific point on stage. They are useful for touring, where lighting designers repeatedly have to rehang, focus, and adjust lights, and automation can help speed the setup process.