Base exposure is the single time, at a fixed aperture and magnification, that renders a negative’s important tones correctly on a given paper and grade. Guessing it wastes paper and chemistry, because print density rises steeply with exposure and the eye cannot reliably predict the result from the projected image. A stepped test strip resolves this by recording several exposures on one sheet, so the correct time is selected rather than estimated.

Aperture and Increment Structure

The enlarging lens is set to a middle aperture for the test, since the central stops resolve detail most evenly across the frame and avoid the diffraction softening of the smallest openings. Ilford’s exposure-testing guidance specifies f/8 as the starting point. Exposure is then stepped in equal one-stop increments by doubling time: Ilford recommends a sequence of 2, 4, 8, 16 and 32 seconds across five bands. Doubling rather than adding fixed seconds keeps each step a constant visual interval, because a stop of light is a doubling regardless of the absolute time. If every band prints too pale the aperture is opened to f/4 and the test repeated; if all are too dark, it is closed to f/16.

Orientation Across the Tonal Range

The strip is laid so its bands cross the negative’s significant tones rather than running through a single flat area such as plain sky. A strip that traverses a textured highlight, a midtone and a shadow lets one exposure be judged against several densities at once, which is what reveals whether a chosen time holds detail throughout. Exposure proceeds by covering successive portions of the paper with an opaque card between increments, so each band accumulates a known total time.

Judging Under Proper Viewing Light

The strip is developed for the paper’s full recommended time, not pulled early, because density continues to build and a short development misreads the exposure. It is then fixed and assessed under normal room light: Ilford notes that after 30 seconds in the fixer the strip can be viewed under normal room lights. Wet prints appear darker than they will once dry, an effect known as dry-down, so the band judged correct is typically one a touch lighter than ideal. The selected band gives the base time; where the correct exposure falls between two bands, a second strip spanning that narrower range refines it.