Enlarger Exposure Calculator (New Print Size, Aperture & Contrast Filter Change)
Convert a known darkroom print exposure to a new print size using the magnification-squared rule, adjust for a different enlarging-lens aperture and a multigrade filter change, and see the result in seconds and f-stops with a table for standard paper sizes.
How it works
magnification M = print long side ÷ negative long side · t₂ = t₁ × ((M₂ + 1) ÷ (M₁ + 1))² × (f₂ ÷ f₁)² × filter factor (×2 for Ilford filters 4–5) · stops = log₂(t₂ ÷ t₁)
When you enlarge more, the same light from the negative is spread over a bigger sheet of paper, and the lens is also farther from the negative, so exposure grows with the square of (magnification + 1) — not simply the square of the print size, though that approximation is close at high magnifications. Changing the lens aperture changes exposure by the square of the f-number ratio, and Ilford's high-contrast filters (4–5) are designed to need twice the exposure of the low grades. This calculator combines the three so your 8×10 exposure predicts the 16×20 without wasting a large sheet.
Worked example
A 35 mm negative printed at 10 in long side (7.1×) needs 12 s at f/8. Going to 16 in (11.3×) at the same aperture and filter: ((11.3 + 1)/(7.1 + 1))² = 2.33, so about 27.9 s (+1.2 stops) — the simple size-squared rule would over-estimate at 2.56×. Opening to f/5.6 halves it to about 14 s, and switching to a grade 4 filter doubles it back.
Frequently asked questions
Why not just use (new size ÷ old size)²?
Because exposure depends on the lens-to-paper distance, which is proportional to magnification plus one, not magnification alone. At 2× to 3× (small prints) the difference is significant; at 10× or more the two rules converge.
Do all multigrade filters need the same exposure?
Ilford Multigrade filters 00 to 3½ are speed-matched; 4, 4½ and 5 need double. Other brands and colour-head dichroic filtration are not matched — test, or use the maker's factor table.
How long is too long an exposure?
Beyond about 60 s papers show reciprocity failure (need a little extra) and the lamp's heat can pop the negative out of focus. Under 5 s is hard to dodge and burn. Adjust the aperture to land between 10 and 30 s.
Related calculators
Darkroom figures are starting points. Film, developer batch, agitation, water and thermometer accuracy all shift results — run a test roll or strip, keep notes, and adjust from your own results.