Film Reciprocity & ND Exposure Calculator
Enter a meter reading, the combined strength of your ND filters, and an ILFORD or Kentmere film. The calculator applies the filter first, then gives the longer timer setting required for the selected film.
Reciprocity-corrected exposure3 minutes 53 seconds
- Nominal time with ND filter
- 1 minute 4 seconds
- Additional reciprocity time
- 2 minutes 49 seconds
- Reciprocity compensation (stops)
- 1.86
- Total increase from metered time (stops)
- 7.86
ND stops are added before the film correction because the reciprocity formula is nonlinear.
Timer values are rounded up to the next millisecond below one second and to the next whole second thereafter.
How to use this calculator
- Enter the meter’s shutter time before fitting an ND filter.
- Add the stop ratings of every ND filter you will stack.
- Select the film loaded in the camera.
- Use the corrected exposure as the timer setting and read the stop figures as a check.
How film reciprocity and ND exposure are calculated
An ND filter reduces the light reaching the film. Each stop halves that light, so every stop doubles the meter’s shutter time. If M is the original metered time and D is the combined ND strength in stops, the nominal filtered time is N = M × 2^D. Add filter ratings together before using the calculator: a 6-stop and a 3-stop filter make 9 stops.
At longer exposures, film does not follow the usual intensity-times-time relationship exactly. ILFORD publishes a film-specific exponent P for its listed black-and-white films. When the nominal time is longer than one second, this calculator uses C = N^P, where C is the corrected timer setting. At one second or less it keeps C = N, following ILFORD’s stated threshold.
The extra reciprocity time is A = C − N. The reciprocity compensation in stops is R = log2(C ÷ N), and the total increase from the original meter reading is S = log2(C ÷ M). Applying the ND filter first matters: raising a time to an exponent is nonlinear, so correcting the meter reading and then adding the filter stops gives a different answer.
What moves the timer setting most
ND strength has an immediate doubling effect: another stop doubles the nominal time before reciprocity is considered. Once that nominal time is long, the selected film’s exponent becomes increasingly important. A film with a larger exponent needs substantially more correction at the same nominal exposure. Stacking filters is fine mathematically, but real filters can transmit slightly differently from their marked ratings.
What this calculator leaves out
This is a timing estimate for the listed ILFORD and Kentmere films, not a universal film rule. It does not account for ND transmission tolerances, color casts, bellows extension, polarizers, unentered filters, changing light, film batch variation, development, or increased scene contrast. At very low light levels, the meter reading itself may be the largest uncertainty, so bracket important photographs.
Worked example
Suppose the meter reads 1 second, you fit a 6-stop ND filter, and you load HP5 Plus. The nominal time is 1 × 2^6 = 64 seconds. HP5 Plus uses P = 1.31, so the corrected time is 64^1.31 = 232.32 seconds; the calculator shows a timer setting of 3 minutes 53 seconds after rounding up.
That adds 168.32 seconds beyond the filtered meter reading. The reciprocity correction is 1.86 stops, and the total increase over the original one-second reading is 7.86 stops.
Common questions
Should ND-filter stops be added before reciprocity correction?
Yes. First multiply the metered time by two for every combined ND stop, then apply the film’s reciprocity exponent to that filtered time. Correcting first and adding ND afterward is wrong because the exponent calculation is nonlinear.
What is the HP5 Plus reciprocity formula?
For nominal exposures longer than one second, ILFORD gives HP5 Plus an exponent of 1.31. The corrected time is the nominal time raised to 1.31: Tc = Tm^1.31. At one second or less, this calculator applies no correction.
How do I combine two ND filters?
Add their marked stop values. For example, a 6-stop filter plus a 3-stop filter is 9 stops, which multiplies the meter reading by 2^9, or 512. Enter 9 as the total ND filter strength.
Why is reciprocity compensation different for each film?
Each emulsion loses effective sensitivity at low light differently. The selected exponent represents ILFORD’s published compensation for that particular film, so a higher exponent produces a larger time increase at the same nominal exposure.
Do digital cameras need reciprocity correction?
No, this is a film-specific calculation and should not be applied to a digital file. Digital long exposures can have different concerns, such as noise, hot pixels, sensor heat, and highlight clipping, but they do not use these film exponents.
Should long exposures be rounded to whole seconds?
For a practical timer, rounding up to a whole second avoids discarding calculated exposure time. This tool keeps millisecond precision below one second and rounds longer displayed times upward. For critical work, test and bracket rather than relying on rounding alone.