Enlarger Exposure Calculator
The new darkroom exposure time after raising or lowering the enlarger for a bigger or smaller print, or after a change in stops.
Result
New exposure time: 36 sEstimate
A starting time, not a final one. Paper batch, developer, filtration and reciprocity failure at long times all shift it, so make a test strip.
- Exposure factor
- 2.25
- Change in stops
- 1.17
https://calcgallery.com/hobbies/enlarger-exposure-calculator/
Results are arithmetic on the figures you enter and are estimates. Products, materials and animals vary, so follow the maker's instructions and label, make a test piece where it matters, and ask a vet about feeding. Nothing here is safety or veterinary advice. Read the full disclaimer
How to use the Enlarger Exposure Calculator
- Enter the exposure time of the print that worked.
- Choose what changed. By lens-to-paper distance, measure from the lens to the paper on the easel before and after.
- By print size, enter one edge of the negative area you print and the same edge on the old and new prints.
- [object Object]
- Use the new time as the middle of a test strip.
How it is calculated
By distance new time = old time × (new distance ÷ old distance)²By print size M = print edge ÷ negative edge new time = old time × ((M new + 1) ÷ (M old + 1))²By stops new time = old time × 2^stopsThe print-size rule and the distance rule are the same rule. By the thin-lens equation, 1/d₀ + 1/dᵢ = 1/f, with magnification M = dᵢ/d₀, the lens-to-paper distance is the focal length × (M + 1). With the same lens at the same aperture, the light on the paper falls with the square of that distance, so the time grows with it squared.
A starting point, not a finished print
The answer holds only if everything else is unchanged: the same lens, aperture, filtration, paper batch, developer and temperature. Change any of them and the time moves. Very long exposures also run into reciprocity failure, where paper needs more light than the rule predicts. Treat the new time as the middle step of a test strip, with steps a third or half a stop apart either side of it.
With your numbers
- Exposure factor = ((11.64 + 1) ÷ (8.47 + 1))² = 1.783
- New time = 16 s × 1.783 = 28.5 s
Quick reference
A starting time, not a final one. Paper batch, developer, filtration and reciprocity failure at long times all shift it, so make a test strip.
| Example | New exposure time |
|---|---|
| 8×10 to 11×14 from 35 mm, 16 s | 28.5 s |
| 8×10 to 16×20 from 4×5, 20 s | 55.6 s |
| 5×7 to 8×10 from 6×6, 12 s | 24.1 s |
| Lens raised from 12 in to 18 in, 16 s | 36 s |
| One stop more from 16 s | 32 s |
| Half a stop less from 16 s | 11.3 s |
| Lens raised from 30 cm to 45 cm, 16 s | 36 s |
| 20 cm to 28 cm print from 35 mm, 10 s | 18.4 s |
Common questions about the Enlarger Exposure Calculator
- Why is the new time not just the ratio of print sizes squared?
- Because the lens moves closer to the negative when you refocus for a bigger print. The light reaching the paper falls with the square of the lens-to-paper distance, which is the focal length times (magnification + 1), not with the square of the magnification alone. Going from an 8×10 to a 16×20 from a 4×5 negative (magnification 2 to 4) needs about 2.8 times the exposure, not 4 times.
- Can I measure the height of the enlarger head instead?
- Only as a rough guide. The rule works on the distance from the lens to the paper. The column scale measures the head, and the lens sits some way below it on its bellows or mount, so head heights give a different ratio, most of all at small prints.
- Why does my bigger print still come out too light?
- Paper loses sensitivity at the low light levels and long times of big enlargements, called reciprocity failure, and the amount differs from paper to paper. The page does not correct for it. Make a test strip around the new time, and expect to add a little at long exposures.
- What about changing the aperture?
- Each full stop on the enlarging lens halves or doubles the light, so opening up one stop halves the time. In the stops method the number is the change in exposure time, not in aperture: enter −1 when you open up one stop and +1 when you stop down one. Use it too for any change you already know in stops, such as a filter factor.
- How do I find the magnification?
- Divide an edge of the print image by the same edge of the negative area being printed. A full-frame 35 mm negative is 24 mm on its short edge, so an 8 inch (203 mm) short edge is a magnification of about 8.5. The page does this for you from the three lengths.