Reciprocity and Equivalent Exposure
Written by Abin Alex and Anu Joseph.
Abstract
Reciprocity is the exchange between light and time: exposure equals the intensity of light multiplied by its duration, so any aperture and shutter speed that multiply to the same value give the same brightness. This guide shows how to find equivalent exposures, what each pair does to the picture, and where the law fails, as in long exposures on film. It also covers the use of ND filters and gives exercises for practice. The central point is that equal exposure does not mean an equal picture. It is written for photographers and cinematographers.
Keywords: reciprocity; equivalent exposure; reciprocity failure; long exposure; ND filter
- What is the law of reciprocity in photography?
- What is equivalent exposure?
- How do I work out an equivalent exposure?
- Do equivalent exposures look the same?
- What is reciprocity failure?
- How do I correct a long exposure on film?
- Do digital cameras have reciprocity failure?
- How do ND filters use reciprocity?
- How can I practise reciprocity?
- How do I use reciprocity in each kind of photography?
What is the law of reciprocity in photography?
Short answer. Reciprocity means that light and time can be swapped against each other. Exposure equals the intensity of the light multiplied by the time it reaches the sensor, so if you let in twice as much light you need only half the time, and the exposure stays the same.
- Intensity is set by the aperture. Time is set by the shutter speed.
- Open the aperture by one stop and make the shutter one stop faster, and the picture has the same brightness.
- The ISO must stay the same while you swap. It does not have to be 100. The law works at any ISO.
- Reciprocity says the pictures are equally bright. It does not say they look alike.
A simple picture to remember: a tap filling a bucket. A strong stream for a short time fills the bucket as much as a weak stream for a long time. The bucket is the sensor. The strength of the stream is the aperture, and the time the tap stays open is the shutter.
Präkel states the law as exposure equals intensity times duration, “within normal sensitivity limits” (Präkel, 2009). The limits are the important part. The law holds at everyday speeds and fails at very long and very short ones, and a later chapter shows where.
Another way to see it: the scene gives you a certain amount of light to work with. The aperture decides how strongly the light arrives, and the shutter decides how long it arrives. You can split the same amount many ways, and the picture has the same brightness. Präkel calls this kind of trade a give and take between the two (Präkel, 2009).
Cham tipThink in stops, not in numbers. One stop more light, one stop less time.
Cham taskSet your camera to manual. Take a picture at f/5.6 and 1/125. Then take one at f/4 and 1/250. Compare the brightness.
Check yourself: You open the aperture from f/8 to f/4. How must the shutter speed change?
Four times faster, because f/8 to f/4 is two stops. For example 1/125 becomes 1/500.
What is equivalent exposure?
Short answer. Equivalent exposures are different combinations of aperture, shutter speed and ISO that produce the same brightness. Whenever you move one setting by a stop, you move another by a stop in the opposite direction.
- One stop is double or half the light. Each f-number on the standard scale is about 1.4 times the one before, because the area of the opening depends on the square of the f-number.
- At a fixed ISO, f/2.8 at 1/500 and f/5.6 at 1/125 are equally bright. The aperture closed two stops and the shutter gave two stops more time.
- Cameras also have third-stop steps. The same idea works with them.
- ISO is the third setting. Doubling the ISO lets you use half the light, so it pays for one stop.
Read the table downwards. Every step closes the aperture by one stop and doubles the time. The last pair, f/32 at 1 s, is ten stops from f/1 at 1/1000, and 1/1000 doubled ten times is just over 1 s.
Two things to know. First, the pairs are equal only to each other. They are right for a scene that needs f/1 at 1/1000 at ISO 100. For a scene that needs f/8 at 1/250, slide the whole table until it matches. Second, the speeds are the rounded numbers on the camera dial, such as 1/60 in place of 1/64.
Präkel gives the same kind of table for a meter reading of EV 10: 1/30 s at f/5.6, 1/60 s at f/4 and 1/125 s at f/2.8 are equal exposures (Präkel, 2009).
| Aperture | Shutter speed | Stops from f/1 |
|---|---|---|
| f/1 | 1/1000 s | 0 |
| f/1.4 | 1/500 s | 1 |
| f/2 | 1/250 s | 2 |
| f/2.8 | 1/125 s | 3 |
| f/4 | 1/60 s | 4 |
| f/5.6 | 1/30 s | 5 |
| f/8 | 1/15 s | 6 |
| f/11 | 1/8 s | 7 |
| f/16 | 1/4 s | 8 |
| f/22 | 1/2 s | 9 |
| f/32 | 1 s | 10 |
Table 1. Eleven equivalent pairs at a fixed ISO. The pairs are worked out for ISO 100.
Cham tipWrite down the pair you started with. Then count stops, not numbers.
Cham taskTake a picture at f/8 and 1/125. Find two other pairs that should give the same brightness, shoot them, and check.
Check yourself: Is f/4 at 1/60 equivalent to f/8 at 1/15, at the same ISO?
Yes. f/4 to f/8 closes the aperture two stops, and 1/60 to 1/15 gives two stops more time. Both pairs are in the table.
How do I work out an equivalent exposure?
Short answer. Count the stops. Whatever number of stops one setting removes, another setting must put back. Aperture, shutter speed and ISO all move in stops, so the changes have to add up to zero.
- Start from a picture that is well exposed.
- Decide what you want to change first: depth of field (aperture), motion (shutter speed) or noise (ISO).
- Count how many stops that change costs. Pay for it with another setting.
- A shortcut at a fixed ISO: the aperture number squared, divided by the time, stays the same. f/8 at 1/125 is 64 times 125, which is 8000. f/4 at 1/500 is 16 times 500, also 8000.
Worked example. A good picture is f/8, 1/125, ISO 100. You want 1/500 to freeze a runner. That is two stops faster. Pay with two stops of aperture and the new picture is f/4, 1/500, ISO 100. If f/4 is too shallow, pay with the ISO instead: f/8, 1/500, ISO 400.
| If you do this | Do this to keep the same brightness |
|---|---|
| Open the aperture 1 stop (f/8 to f/5.6) | Make the shutter 1 stop faster (1/125 to 1/250), or lower the ISO 1 stop |
| Close the aperture 2 stops (f/4 to f/8) | Make the shutter 2 stops slower (1/500 to 1/125), or raise the ISO 2 stops (100 to 400) |
| Make the shutter 3 stops faster (1/60 to 1/500) | Open the aperture 3 stops (f/8 to f/2.8), or raise the ISO 3 stops (100 to 800) |
| Raise the ISO 2 stops (100 to 400) | Close the aperture 2 stops, or make the shutter 2 stops faster |
Table 2. Paying for a change, one stop at a time.
Cham tipChange the setting you care about first. Then pay with the one you care about least.
Cham taskPhotograph a moving subject at 1/125, then at 1/500. Change the aperture to keep the brightness the same. Which picture freezes the movement?
Check yourself: A good picture is f/5.6, 1/60, ISO 100. You raise the ISO to 400. What shutter speed keeps the brightness, with the aperture still at f/5.6?
1/250. ISO 400 is two stops more sensitive, so the time must be two stops shorter.
Do equivalent exposures look the same?
Short answer. No. The brightness is the same, but each setting changes a different part of the picture. Aperture changes the depth of field, shutter speed changes the motion blur, and ISO changes the noise.
- A wide aperture gives a shallow depth of field. A very small aperture can soften the picture through diffraction.
- A fast shutter freezes movement. A slow shutter blurs moving subjects and shows camera shake.
- A high ISO means more noise.
- So choose the pair for the picture you want, not out of habit.
Cham tipBefore you press the shutter, say which of the three you are protecting: focus, motion or noise.
Cham taskShoot a person at f/2, f/5.6 and f/11, changing the shutter speed to keep the brightness. Look at the background and at the hands.
Check yourself: You need a sharp background and a frozen action in dim light. Which setting do you pay with?
The ISO. A higher ISO gives you the extra light without changing the depth of field or the blur, but the picture will be noisier.
What is reciprocity failure?
Short answer. Reciprocity failure is the point where the exchange stops working. On film, very long exposures (a second or more) and very short ones (around 1/10,000 s or less) leave less of a mark than the law predicts. The picture comes out darker than expected, so you must add exposure.
- In the middle range of times, 1/1000 s to 1/10 s for Kodak Tri-X 400, the law holds and no correction is needed (Kodak, n.d.).
- For long exposures Tri-X 400 asks for one extra stop at 1 s, two at 10 s and three at 100 s (Kodak, n.d.).
- Very short exposures need extra light too: half a stop at 1/10,000 s and one stop at 1/100,000 s (Kodak, n.d.).
- Each film behaves differently, so read the data sheet of the film you use.
| Indicated time | Open the lens | Or give this time | Change development |
|---|---|---|---|
| 1/100,000 s | +1 stop | +20% | |
| 1/10,000 s | +½ stop | +15% | |
| 1/1000 s | none | +10% | |
| 1/100 s | none | none | |
| 1/10 s | none | none | |
| 1 s | +1 stop | 2 s | −10% |
| 10 s | +2 stops | 50 s | −20% |
| 100 s | +3 stops | 1200 s | −30% |
Table 3. Kodak’s adjustments for Tri-X 400 film (Kodak, n.d.).
Cham tipOn film, do not trust the meter reading past one second. Look at the data sheet.
Cham taskFind the data sheet of a film you know. Note the exposure time where it starts to ask for extra exposure.
Check yourself: Is reciprocity failure a fault of your camera?
No. It is a property of the film. The camera works as normal, but the film responds less to a weak light acting for a long time.
How do I correct a long exposure on film?
Short answer. Raise the metered time to a power that is specific to the film. Ilford writes this as Tc = Tm to the power P, where Tm is the metered time and Tc the corrected time. For HP5 Plus P is 1.31, so a metered 10 s becomes about 20 s (Ilford Photo, n.d.).
- P is a number from the film maker. It is different for every film.
- Ilford says exposure times of one second or less need no compensation (Ilford Photo, n.d.).
- Work in seconds and use a calculator with a power key.
- Worked example: HP5 Plus, metered 30 s. 30 to the power 1.31 is about 86 s, which is one and a half stops more.
| Ilford film | P | 10 s becomes | 60 s becomes |
|---|---|---|---|
| FP4 Plus | 1.26 | 18 s | 174 s |
| HP5 Plus | 1.31 | 20 s | 213 s |
| Pan F Plus | 1.33 | 21 s | 232 s |
| Delta 400 | 1.41 | 26 s | 322 s |
| SFX | 1.43 | 27 s | 349 s |
Table 4. P values from Ilford and the times they give. The times are worked out from the formula (Ilford Photo, n.d.).
Cham tipWork out the corrected time before you open the shutter and write it in your log.
Cham taskWork out the corrected time for a metered 15 s on HP5 Plus. Check your answer: it should be near 35 s.
Check yourself: The meter says 0.5 s on HP5 Plus. Do you correct it?
No. Ilford says one second or less needs no compensation.
Do digital cameras have reciprocity failure?
Short answer. Not in the film sense. A sensor gathers light as electric charge and has no emulsion. Long exposures cause a different problem: noise increases, including bright specks and banding, and the camera may spend extra time cleaning the image up.
- Film responds less to weak light acting for a long time. A sensor does not have that chemical problem.
- Long exposures are noisy. Canon’s long exposure noise reduction works on shots of 1 s or longer (Canon, n.d.).
- The camera can take about as long to process the picture as the exposure itself, and you cannot shoot again until it finishes.
- Turn it on or off by testing. Compare a 30 s picture with and without it.
Cham tipTest your own camera once. Shoot 30 s with noise reduction off and on, and look at the shadows.
Cham taskShoot a dark room for 30 s with long exposure noise reduction on and off. Zoom in on the dark areas of both.
Check yourself: Why does a 30 s picture take extra time to appear when long exposure noise reduction is on?
The camera is processing the picture to reduce noise. Canon says this can take about as long as the exposure itself.
How do ND filters use reciprocity?
Short answer. An ND (neutral density) filter reduces the light by a known number of stops without changing the colour. In bright light it lets you use a longer shutter time or a wider aperture. Each stop of filter doubles the time.
- ND 0.3 is one stop, ND 0.6 is two stops and ND 0.9 is three stops.
- A 10 stop filter turns 1/125 s into about 8 s.
- Meter with the filter on, or work out the time from the table.
- On film, add the reciprocity correction if the new time goes past one second.
| ND density | Stops | Light passed | 1/125 s becomes |
|---|---|---|---|
| 0.3 | 1 | 1/2 | 1/60 s |
| 0.6 | 2 | 1/4 | 1/30 s |
| 0.9 | 3 | 1/8 | 1/15 s |
| 1.2 | 4 | 1/16 | 1/8 s |
| 1.8 | 6 | 1/64 | 1/2 s |
| 3.0 | 10 | 1/1000 | 8 s |
Table 5. Common ND filters. A density of 0.3 is one stop. Ten stops is a factor of 1024, shown as 1000.
Cham tipPut the filter on after you focus. A dark filter makes autofocus slow.
Cham taskIf you have an ND filter, shoot a moving cloud or water at 1/60 s and then with the filter. Work out the new shutter speed first.
Check yourself: A scene needs 1/250 s at f/8. You add a 3 stop ND. What shutter speed keeps the brightness?
1/30 s. Three stops is eight times more time, and 1/250 times 8 is about 1/30.
How can I practise reciprocity?
Short answer. Photograph one still subject five times, using five pairs of settings that should be equally bright. Then compare the brightness, the depth of field and the blur. If the brightness differs, your stop count was wrong.
- Use a tripod and manual mode. Keep the ISO fixed.
- Meter once and note the settings. Then move the aperture one stop at a time and fix the shutter each time.
- Put a moving object in the frame, such as a fan or a hand, so you can see the blur change.
- Fill in the log. The numbers teach more than the pictures.
| Aperture | Shutter speed | ISO | Brightness | What changed |
|---|---|---|---|---|
Table 6. Practice log. Copy this into your notebook.
Cham tipWrite the stop count next to every change. If the count is wrong, you will see it in the log.
Cham taskDo the five-picture round today in daylight and post your log and your pictures.
Check yourself: Why must the ISO stay fixed in this exercise?
If the ISO changes, it adds a third trade. Then you cannot tell which setting changed the brightness.
How do I use reciprocity in each kind of photography?
Short answer. Reciprocity lets you swap light and time at everyday speeds, so one stop more aperture buys one stop faster shutter. It fails only in very long exposures on film. Treat every line below as a starting point.
- Birds. Open the aperture one stop to double the shutter speed and freeze a wing.
- Animals and pets. Open one stop, and the shutter speed can double when the animal moves.
- Portraits. In bright sun at a wide aperture, use an ND filter instead of closing the aperture. The background stays soft.
- Fashion. With flash, the aperture and ISO set the flash exposure, and the shutter speed mainly sets the ambient light.
- Wedding and events. In a dim hall, open one stop or raise ISO one stop to keep the same brightness at the same shutter speed.
- Product. On a tripod you can trade time for light freely. Keep the lowest ISO and the aperture you want.
- Automotive. For light trails, use 10 to 30 s at f/8 and the lowest ISO. A digital camera needs no film correction.
- Sports and action. Open the aperture one stop and the shutter speed can double, for example from 1/1000 s to 1/2000 s.
- Landscape. A 10 stop ND filter (ND 3.0) turns a daytime scene into a long exposure. Count the stops.
- Night and astro. On film, add extra time for reciprocity failure using the P factor. On a digital camera, long exposure noise reduction doubles the waiting time.
- Cinema. The shutter angle is fixed, so ND filters take the place of a faster shutter. Each ND 0.3 is one stop.
Questions beginners ask
What is the law of reciprocity?
Exposure equals the intensity of light times the time it falls on the sensor. If you double one and halve the other, the exposure stays the same.
Does reciprocity only work at ISO 100?
No. It works at any ISO, as long as the ISO does not change while you swap the aperture and the shutter speed.
What is an equivalent exposure?
A different combination of aperture, shutter speed and ISO that gives the same brightness. For example, f/4 at 1/250 and f/8 at 1/60 at the same ISO.
What is reciprocity failure?
It is when film responds less than the law predicts, in exposures of about a second or longer and in very short ones. You must add exposure to correct it.
Do digital cameras have reciprocity failure?
Not in the film sense. In long exposures the problem is noise, and many cameras offer long exposure noise reduction.
References
- Canon. (n.d.) Long Exposure Noise Reduction. EOS R6 Mark II Advanced User Guide. cam.start.canon/en/C012/manual/html/UG-04_Shooting-1_0290.html
- Ilford Photo. (n.d.) Reciprocity failure and compensation. Film data sheet with P factors. ilfordphoto.com/amfile/file/download/file/1964/product/695
- Kodak. (n.d.) Kodak Professional Tri-X 320 and 400 Film. Data sheet F-4017. kodakprofessional.com/sites/default/files/wysiwyg/film/f4017_trix_320400.pdf
- Präkel, D. (2009) Basics Photography 07: Exposure. Lausanne: AVA Publishing. See the sections on stops (pp. 26–27) and reciprocity (pp. 28–29).
How to cite this guide. Alex, A., & Joseph, A. (2026). Reciprocity and equivalent exposure: A guide for photographers and cinematographers. Creative Hut Academy. https://creativehut.org/library/reciprocity-and-equivalent-exposure/
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