What Is Reciprocity in Photography? Easy Explanation & Examples
Have you ever wondered why the same beam of light can sometimes turn a photo into a dreamy blur, yet other times freeze every single strand of hair in perfect detail? The answer lies in a simple, almost magical rule in photography: the reciprocity law.
So what is the reciprocity law? In this article, we'll explain it in the clearest, most down-to-earth way so you can quickly grasp this somewhat technical, but often confusing, photography concept.
In this article, you will learn:
What Is Reciprocity?
So what is reciprocity? Simply put, it's a kind of equation, a fixed relationship that ties several exposure factors together.
To explain it clearly, we need to start with how bright or dark a photo is.
When you take a picture, aside from framing, focusing, and composition, the most important job is controlling the photo's brightness — in other words, its exposure.
A photo's brightness depends on four things: the strength of the ambient light, the aperture, the shutter, and the ISO (sensor sensitivity).
- Ambient light is the amount of light in the scene you're shooting. The stronger the light, the easier it is to make the photo bright.
- Aperture is the hole in your lens. It can be large or small. A larger aperture lets in more light; a smaller one lets in less. Bigger apertures make it easier to get a brighter photo.
- The shutter works like a gate. It controls how long the sensor is exposed. The longer it's open, the more light gets in. A slower shutter speed (longer exposure) makes the photo brighter.
- ISO measures how sensitive the sensor is to light. A higher ISO will make the same amount of light look brighter. Higher ISO makes it easier to get a bright photo.
If that still sounds confusing, don't worry. We'll explain aperture, shutter, and ISO in more detail, because those three are the settings you can change on your camera.
1. What Is Aperture?
Aperture is the adjustable hole inside your lens. Changing the size of that hole changes how fast light passes through.
- Big hole = large aperture = more light in per second.
- Small hole = small aperture = less light in per second.
One important thing to remember: the smaller the f-number, the larger the aperture. And the larger the f-number, the smaller the aperture.
Common full-stop f-numbers are: f/32, f/22, f/16, f/11, f/8, f/5.6, f/4, f/2.8, f/2, f/1.4, f/1.0.
This list above shows f-numbers from large to small, so the aperture itself goes from small to large. For example, we usually call f/2.8 (and lower numbers) a large aperture, and f/8 (and higher numbers) a small aperture.
That also means the list goes from darker to brighter: smaller apertures let in less light; larger apertures let in more.
Each full stop is related by a factor of √2 in f-number, which makes the lens's light-admitting area change by 2×.
So under the same conditions, opening the aperture by one full stop makes the image twice as bright; closing it by one stop makes the image half as bright.
2. What Is the Shutter?
The shutter is a gate in front of your camera's sensor. When the gate opens, exposure starts. When it closes, exposure stops. The longer the shutter is open, the more light reaches the sensor.
Common full-stop shutter speeds are: 1/8000s, 1/4000s, 1/2000s, 1/1000s, 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s, 1/8s, 1/4s, 1/2s, 1s, 2s, 4s, 8s, 15s, 30s.
This list goes from short (darker) to long (brighter) exposures.
Each full stop doubles or halves the exposure time. So, under the same conditions, slowing the shutter by one stop makes the image twice as bright. Speeding the shutter by one stop makes the image half as bright.
If you open the aperture by one stop, the lens lets in twice as much light, so the image becomes one stop brighter. If you then make the shutter one stop faster (half the time), the image becomes one stop darker.
Those two changes cancel each other out — overall exposure stays the same.
In general, if you move the shutter N stops toward brighter (slower), and move the aperture N stops the opposite way (smaller), the overall exposure won't change.
3. What Is ISO?
ISO measures your camera sensor's sensitivity to light. A higher ISO number means the sensor is more "sensitive."
Common full-stop ISO values are: ISO 50, ISO 100, ISO 200, ISO 400, ISO 800, ISO 1600, ISO 3200, ISO 6400, ISO 12800, ISO 25600.
Each full stop doubles the sensor's sensitivity. In other words, each step is 2× the previous one. So the list above goes from darker to brighter.
Under the same lighting, raising ISO by one stop makes the image twice as bright. Lowering the ISO by one stop makes the image half as bright.
4. Conclusion: What Is Reciprocity in Photography?
If one or two of aperture, shutter speed, or ISO are changed by N stops toward brighter (or darker), the other one or two can be changed by N stops the opposite way, and the overall exposure stays the same.
Reciprocity is the rule that aperture, shutter speed, and ISO can make up for each other. Different combinations of these three can give you the same exposure.
How to Understand Reciprocity Better?
Let's use a simple analogy. Imagine a cylindrical water tank with a 1 m² base and 2 m depth. Mark the tank at 1 m. Your goal is to fill the tank to that 1 m mark.
Ambient light is like the water pressure. Brighter light = more pressure. Usually, you can't change the ambient light, so we'll ignore it.
Aperture is the faucet. Open it wider, and more water flows per second. Shutter speed is how long you turn the faucet on.
Say the faucet (aperture) is set so it pours 1 cubic meter per second (think: f/2.0). If you open the faucet for 1 second (shutter = 1s), you'll reach the 1 m mark.
Now empty the tank and try again. If the faucet now pours 0.5 cubic meters per second (think: f/2.8), you need 2 seconds of flow (shutter = 2s) to reach the same 1 m mark.
That's reciprocity.
Let's extend the example with ISO. If you raise ISO by one stop (for example, ISO 100 → ISO 200), it's like shrinking the tank base from 1 m² to 0.5 m². With the faucet still pouring 1 cubic meter per second, you now only need 0.5 seconds of flow to hit the 1 m mark.
We can summarize with a simple formula for intuition (not a precise physics equation):
- Photo brightness = ambient light × aperture × shutter speed × ISO.
Each stop change in aperture, shutter, or ISO changes brightness by a factor of 2. So the different controls are interchangeable in that way.
Examples of how to keep exposure the same:
If you close the aperture by 2 stops, you can keep the exposure unchanged by:
- Slowing the shutter by 2 stops, or
- Slowing the shutter 1 stop and raising ISO 1 stop, or
- Raising ISO 2 stops.
If you make the shutter 4 stops faster (less light), you can keep exposure the same by:
- Opening the aperture 4 stops, or
- Opening aperture 3 stops and raising ISO 1 stop, or
- Opening aperture 2 stops and raising ISO 2 stops, or
- Opening aperture 1 stop and raising ISO 3 stops, or
- Raising ISO 4 stops.
The rule is simple: if one control moves N stops in one direction, the other controls combined must move N stops the opposite way to keep exposure the same. That's reciprocity.
Conclusion
In the era of automatic cameras, we don't calculate this by hand very often. In P, A (Av), and S (Tv) modes, the camera will change the other settings automatically as long as your exposure compensation stays the same. Even in Manual mode, the camera's meter and the exposure scale show you the equivalent changes.
Still, knowing reciprocity deeply helps you understand exposure. It lets you pick a specific aperture or shutter speed you want and then adjust the other settings to get the correct exposure.
For example, if you're shooting a landscape and your starting settings are f/2.8, 1/2000s, ISO 100 with 0 EV, but you want a smaller aperture (f/8), reciprocity tells you to slow the shutter by 3 stops to keep the same exposure, which lands you at about 1/250s.
And the camera's meter will show this for you, so you don't have to do the math.
By the end of this article, you should now understand what reciprocity in photography is. It's not deep or complicated. It's mostly just simple math.
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