A test strip is usually made using a strip of the photographic paper to be used for a full-size print. The paper should be fixed firmly in place so that id cannot move during the exposure process. A range of exposures is then used on various parts of the strip to determine the optimum figure before committing to the full print.
A timer is set to a period determined by experience of using a particular enlarger, and the relevant magnification (the distance from the lamp to the paper remembering the inverse square law). An aperture of f/11 or f/8 and an interval of five seconds might be typical when enlarging a medium-format negative to A4 size. In the example above, the timer was set to 15 seconds and the test strip was exposed for the first time. Three-quarters of the test strip was then covered with card before making a second 15-second exposure. This process was repeated this until the last quarter of the test strip was covered. The result was four zones exposed for 15, 30, 45 and 60 seconds.