light. Such enlargers can also be used with variable-contrast monochrome papers. Digital enlargers project an image from an LCD screen at the film plane, to produce a photographic enlargement from a digital file. Enlarger physical arrangements Most modern enlargers are vertically mounted with the head pointing downward and adjusted up or down to change the size of the image projected onto the enlarger's base, or a work table if the unit is mounted to the wall. A horizontal enlarger consists of a trestle, with the head mounted on crossbars between two or more posts for extra stability. A horizontal enlarger structure is used when high quality, large format enlargements are required such as when photographs are taken from aircraft for mapping and taxation purposes. The parts of the enlarger include baseboard, enlarger head, elevation knob, filter holder, negative carrier, glass plate, focus knob, girder scale, timer, bellows, and housing lift. Principles of operation The image from the negative or transparency is projected through a lens, typically fitted with an adjustable aperture, onto a flat surface bearing the sensitized photographic paper. By adjusting the ratio of distance from film to lens to the distance from lens to paper, various degrees of enlargement may be obtained, with the physical enlargement ratio limited only by the structure of the enlarger and the size of the paper. As the image size is changed it is also necessary to change the focus of the lens. Some enlargers, such as Leica's "Autofocus" enlargers, perform this automatically. An easel is used to hold the paper perfectly flat. Some easels are designed with adjustable overlapping flat steel "blades" to crop the image on the paper to the desired size while keeping an unexposed white border about the image. Paper is sometimes placed directly on the table or enlarger base, and held down flat with metal strips. The enlargement is made by first focusing the image with the lamp on, the lens at maximum aperture and the easel empty, usually with the aid of a focus finder. The lamp is turned off, or in some cases, shuttered by a light-tight mechanism. The image is focused by changing the distance between the lens and the film, achieved by adjusting the length of a light-tight bellows with a geared rack and pinion mechanism. The lens is set to its working aperture. Enlarging lenses have an optimum range of apertures which yield