Darkroom Enlarger Exposure Calculator – New Print Time When You Raise the Enlarger Head

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Why raising the enlarger changes the print time

When you raise the enlarger head to make a bigger print, the same amount of light spreads over a larger area, so each part of the paper receives less light per second. To get the same darkness, you must expose for longer. The rule is the inverse square law in plain form: double the height and the print needs four times the exposure, because the area grows with the square of the height. This surprises many beginners, who expect a doubled height to need a doubled time. It needs much more, and the calculator above does that squaring for you.

Enter the print time that worked at your old height, the old height, and the new height. The tool returns the new time at the same aperture, and a second time if you also change the aperture. For example, a 20 second print at 30 centimetres becomes an 80 second print at 60 centimetres. If you also open the aperture by one stop, that 80 seconds drops back to 40 seconds, because each stop of aperture doubles or halves the light. This second number is handy when a long exposure feels awkward, since very long exposures make dodging and burning harder to time.

How to measure enlarger height correctly

Measure from the lens to the paper, or from the negative to the paper, and use the same two points for both measurements. What matters is the ratio between the two heights, not the absolute number, so a consistent method beats a perfect one. If your enlarger column has a scale, use it for both readings. If you measure with a tape, measure the same way twice. A small error in the ratio gets squared in the result, so a careless measurement can push your print time noticeably off. When the new height is only a little larger, the change is small and forgiving. When you jump from a small print to a large one, measure twice.

Aperture changes deserve care too. Opening up by one stop halves the time but also softens the image slightly, because most enlarging lenses are sharpest two or three stops down from wide open. Closing down sharpens the image but lengthens the time. If your new time is very short, under about 8 seconds, dodging and burning become hard to control, so consider closing down a stop. If your new time is very long, over about 60 seconds, reciprocity and impatience both become problems, so consider opening up. The calculator shows both numbers so you can pick the balance that suits your print.

Test strips still decide the final print

The calculator gives a starting time, not a finished print. Paper batches vary, developer strength drops as it ages, and your old time may itself have been a compromise. Always make a test strip at the calculated time, with steps around it, before committing a full sheet of large paper. Large paper is expensive, and a test strip on a small offcut costs almost nothing. Expose strips at the calculated time minus 25 percent, at the calculated time, and at the calculated time plus 25 percent, then pick the best and refine from there.

Record the winning combination: height, aperture, time, paper, developer, and temperature. Darkroom work is repeatable only when it is written down. The next time you print the same negative at the same size, your notes give you the answer in seconds instead of another round of test strips. The calculator is the shortcut to the first good test strip, and your notebook is the shortcut for every print after that.

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