Stouffer Step Wedge Exposure Calculator
Enter the exposure time from a washed-out Stouffer 21-step test and the highest fully retained step. The calculator returns the corrected exposure, whether the test was under or over target, and a practical retry bracket.
Corrected exposure time120
- Exposure correction factor
- 2
- Exposure time change
- 100%
- Step difference
- 2
- Exposure diagnosis
- Underexposed
- Suggested retry bracket
- 84.9 seconds to 169.7 seconds around 120 seconds
Assumes a Stouffer-style 21-step guide with 0.15 density, or one-half stop, between adjacent steps.
Use the highest fully retained step after normal washout; partially retained steps are not counted.
How to use this calculator
- Expose, wash out, and dry the test screen using the same process you plan to use for the retry.
- Enter the original exposure time in seconds.
- Enter the highest numbered step that is fully retained, not a partly held step.
- Leave the target at step 7 unless your emulsion maker or shop standard says otherwise.
- Choose the retry bracket span to test times around the calculated exposure.
How the corrected exposure is calculated
A Stouffer 21-step transmission guide changes by 0.15 optical density at each step. For the common T2115 guide, that is one-half stop per step. This calculator uses that half-stop spacing to turn the difference between the step you held and the step you wanted into an exposure multiplier.
Let E be the test exposure in seconds, O the observed highest fully retained step, T the target retained step, and B the retry bracket span in steps. The step difference is D = T - O. The correction factor is:
F = 2^(D / 2)
The corrected exposure is C = E * F. The percentage change is 100 * (F - 1). The retry bracket is centered on C, with L = C * 2^(-B / 2) and U = C * 2^(B / 2).
If the observed step is below the target, the test is underexposed and needs more time. If the observed step is above the target, the test is overexposed and needs less time. If the two steps match, the exposure time is already on target for the conditions you tested.
What changes the result most
The step difference is the main driver because two steps equal one full stop. A two-step miss doubles or halves the exposure. A four-step miss changes the exposure by a factor of four, which is large enough that a fresh bracketed test is better than treating the new time as final. The original exposure time scales the answer directly: the same two-step correction turns 30 seconds into 60 seconds, or 90 seconds into 180 seconds.
How to read the step
Use the highest step that is fully hardened after normal washout. Do not count a step that is only partly visible, weak at the edge, or held because the washout was gentler than normal. A reading at step 1 or step 21 can be censored by the end of the wedge, meaning the true result may be beyond what the guide can show.
What this calculator leaves out
The calculation only corrects a physical test. It cannot predict a reliable starting exposure without an actual wedge reading. It also assumes the same mesh, emulsion, coating thickness, positive density, lamp distance, lamp output, drying, and washout method between the test and retry. Changing any of those can move the target time enough to require a new test.
Worked example
Suppose you exposed a test screen for 60 seconds and, after normal washout, the highest fully retained step is 5. With the target left at step 7, the step difference is 7 - 5 = 2.
The correction factor is 2^(2 / 2) = 2, so the corrected exposure is 60 * 2 = 120 seconds. The exposure change is 100 * (2 - 1) = 100%, and the diagnosis is underexposed. With a one-step retry bracket, the suggested times are about 84.9 to 169.7 seconds around 120 seconds.
Common questions
How do I read a Stouffer 21-step wedge after washout?
Read the highest numbered step that is fully retained after your normal washout. Ignore steps that are faint, partly broken, or only held at an edge. Consistent washout matters because the calculator assumes the reading reflects exposure, not a different rinse technique.
Why is step 7 the usual exposure target?
Stouffer describes step 7 as the normal-copy aim point for a 21-step guide in plate work, with the next step only slightly held. Screen-printing shops often use the same aim unless the emulsion maker or an internal standard specifies another retained step.
Does a higher retained step mean too much exposure?
Yes, relative to a lower target. More exposure hardens farther into the denser steps, so an observed step above the target points to overexposure and a shorter retry time. An observed step below target points to underexposure and a longer retry time.
Why does a two-step difference double or halve exposure?
A 21-step Stouffer guide has 0.15 density per step, which corresponds to one-half stop. Two steps therefore equal one stop. One stop is a factor of two in exposure time.
Should I retest after calculating the corrected exposure?
Usually yes. The calculation gives the center point for the next test, not proof that every shop variable stayed ideal. A bracket one step below and above the calculated time is a practical way to confirm the result.
Can this predict an initial exposure time without a wedge test?
No. It corrects a known exposure from a physical test reading. For a first exposure, use your emulsion guidance or shop history, then run the wedge test and enter the result here.