Bellows Extension Factors

To focus on an object very close to the lens, it is necessary to increase the distance between the lens and the film. This is important to know because if the film - lens distance becomes greater than the focal length of the lens, the f/stop numbers become inaccurate.

An interesting way to visualize why this happens is to imagine that you're inside a large camera that is focused on an object approaching the lens. If the lens is set at f/11, as the lens gets farther away from the film, the size of aperture f/11 will appear to decrease. Eventually, f/11 will appear to be the size of f/22 from the film's point of view. The rule is: As the lens-to-film distance increases, the effect of a given f/stop will decrease. Unless you compensate for this, the negative will be underexposed when you use extension tubes on a 35 mm camera or extend the bellows of a view camera beyond the focal length of the lens.

The bellows extension factor tells you how much exposure to add to get good results when doing close-up photography. There is a mathematical formula that can be used to compute this factor, but an easier way to figure it out is to use one of the many handy devices that have been developed especially for this purpose. Two examples are the Quick Stick, manufactured by Visual Departures in New York, and the Calumet Exposure Calculator.

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