The Reciprocity Rule in Photography

As photographers, we are constantly bound by the need to heed certain unalterable constants that define our images. For example, a thorough understanding of focal length goes a long way as, no matter what kind of lens you use, the way focal length affects your photos is consistent across all cameras, sensor formats, and image types.

Likewise, there is the principle of reciprocity in photography. Many claim this to be one of the most crucial technical concepts to grasp as an aspiring beginner. But what exactly is the law of reciprocity in photography? And is it truly as clear-cut as the name makes it sound?

Today, I will be examining just that in this comprehensive overview of reciprocity in photography and all there is to know about it.

What is Reciprocity?

Let’s start with the basics. What does the principle of reciprocity aim to explain?

In simple terms, reciprocity is a direct and inverse relationship between multiple factors. Increase the value on one end, and the value on the other end drops. Vice versa, the same applies of course. Think of a seesaw, and you get the broad idea.

In a very similar fashion, the so-called law of reciprocity in photography describes the relationship between your main exposure controls – namely shutter speed and aperture – and exposure times.

In a nutshell, exposure being in a reciprocal relationship means that opening the aperture by one stop (i.e. from f/2 to f/3.5, for example) is equivalent to lowering the shutter speed by the same amount (such as from 1/100 to 1/50). The inverse relationship, of course, also holds true the same way and vice versa.

Reciprocity is such a powerful principle because it allows you to handle exposure time holistically. Instead of asking yourself, ‘What aperture settings do I need for this shot?’, you can change the question to, ‘What EV (exposure value) am I going for?’.

When you realize that any single EV can be represented by a wide variety of aperture or shutter speed settings, then you will see how reciprocity allows you to achieve much more diverse-looking photographs than you may otherwise have thought possible.

Photographic Stops and How They Relate to Reciprocity

For reciprocity to work out, we need some kind of universal unit to compare exposure settings on equal terms. After all, shutter speed is measured in fractions of a second. Meanwhile, the common notation for lens apertures are f-numbers – fractions of the aperture width and focal length.

With these kinds of vastly different units in place, how do you honor reciprocity? How do you make sure as a photographer that you truly increase or decrease both values by the same amount?

The answer lies in stops. In photography, stops are the unit we use to compare all exposure settings under equal terms. This does not just include shutter and aperture, mind you, but also ISO numbers!

One ‘stop’ of exposure expresses the same amount of light passing through the lens opening, no matter which setting you manipulate to add or remove that stop. For example, moving from f/11 to f/8 is a difference in EV of one stop. The same goes for doubling your shutter speed from 1/25 to 1/50. Take away a stop on one setting and add it with another – this is reciprocity made easy!

While the principle is simple, defining a stop in hard numbers is actually a very involved affair. That a shutter speed of 1/25 lets in twice as much light as 1/50 of a second seems reasonable enough. But how is it that the difference in light between f/11 and f/8 and between f/2 and f/2.8 is equal?

The answer lies in the shape of the aperture as a geometric circle. Since the area of a circle is governed by the equation pi*r², aperture f-numbers do not correlate with the amount of light they represent in a linear fashion.

Fortunately for the mathematically challenged, pretty much all modern cameras and lenses automatically compensate for this. Whether apertures on the lens ring or shutter speed in-camera, everything is incremented in whole, halves, or sometimes even thirds of a stop.

This means that you don’t need to worry about doing math in your head to figure out how many stops a change in aperture equates to in order to dial in the appropriate shutter speed. Rather, one ‘click’ on your shutter speed dial is guaranteed to correspond to one ‘click’ of the aperture ring in any direction!

Same Exposure, Different Shutter Speeds

Try this easy experiment to put reciprocity to the test.

Pick a subject and scene of your choice. Preferably do this in an outdoor setting where the availability of ambient light won’t be an issue. Now, guided by your camera’s light meter, choose a combination of aperture and shutter speed settings that gives you a satisfying exposure.

After taking the shot in this way, go into manual mode. Try to assess the scene and change the shutter speed to a different value than what you just used. Now, assess how you will need to adapt the aperture setting to compensate. Take the same shot again.

For maximum effect, let the camera stand in place on a tripod so the composition does not change.

How many differences can you spot despite the equivalence in EVs between the two shots? Which do you prefer? Keep going and take and re-take more attempts in the same manner to figure out where your sweet spot lies.

Choosing Shutter Speed with the Reciprocal Rule

Now, you should have a basic grasp of what reciprocity means in the context of achieving pleasing exposure. However, today’s lesson does not stop there.

Reciprocity has many more applications in live photography than merely allowing you to intelligently combine shutter speeds and aperture settings.

For example, consider the reciprocal rule, widely taught in many photography and film schools.

The reciprocal rule states that you should always set your shutter speed to the value of ‘one over focal length’ or faster. For example, when wielding a 50mm lens, your slowest shutter speed under the reciprocal rule should be 1/50.

How the Reciprocal Rule Works

Let’s take a look at the rationale behind this rule.

The basic guideline that the reciprocal rule expresses is the inverse relationship between focal length and camera shake. In other words, a longer lens magnifies more strongly, rendering small involuntary movements more grave in the final image.

Thus, increases in focal length require a corresponding increase in shutter speed to offset the visible shake and mitigate blur.

Note that this inversely proportional relationship is not the same as I described in the general definition of reciprocity above. Reciprocity in photography does not describe one specific relationship. Rather, it’s the general nature of reciprocal relationships between exposure factors overall that matters.

Put another way, reciprocity can take many forms, and this reciprocal rule merely concerns one of these. Hence, you may, in other contexts, also see entirely different relationships between other exposure factors referred to as ‘the reciprocal rule’.

Shortcomings of the Reciprocal Rule

As for the rule above, note that it comes with a few limitations. For one, steadying your camera on a tripod removes the need to adhere to minimal shutter speed settings.

A stabilized camera can shoot photos at a much slower shutter speed without incurring negative effects of shake or blur. Hence, the ‘one over focal length’ rule is only valid when shooting handheld.

There is also the issue of sensor size. The ‘one over focal length’ paradigm, where it does hold true, is intended for use with 35mm full-frame sensors. Larger or smaller sensors alter the angle of view, which is akin to zooming in or out. This completely changes the equation since the apparent camera shake will be visible at different thresholds.

Applying the rule to an MFT (Micro 4/3) sensor, for instance, results in a baseline of ‘two over focal length’ instead. This is because the MFT format has a crop factor of 2.0 in reference to 35mm full-frame.

Reciprocity Failure and Why It Matters

Knowing about the principles behind reciprocity and how ubiquitous they are in photography can feel very powerful. Suddenly, you have access to an entire framework for judging exposure time and settings accurately without any tools!

It’s not that simple, unfortunately. There are certain cases where one kind of reciprocity may break down. As a whole, this is known as reciprocity failure.

The most significant case of reciprocity failure applies to the reciprocal relationship between shutter speed and aperture. Let’s talk about how that works.

When a Slower Shutter Speed Isn’t Enough

In everyday photography, the law of reciprocity is an incredibly useful tool that you can use to find the combination of shutter speed and aperture setting inputs that you need for your shot.

The greatest thing about reciprocity is that it allows you to make adjustments to one factor, compensate with the other factor, and still end up with the same exposure. At the same time, the variances between exposure factors can create creatively unique images.

Under certain circumstances however, reciprocity breaks down. This mostly happens when taking long exposures, and in particular in film photography.

At an exposure time of multiple seconds or longer, the amount of light that you need to get equivalent exposures to a shot taken with a wider aperture-shutter speed combination dramatically increases. The linear and inversely proportional relationship simply stops working, in other words. In such a scenario, you need to make manual adjustments to keep getting accurate exposures.

Fundamentally, this happens because the chemicals that give the film its sensitivity to light can actually become exhausted. When this happens during a longer exposure, the overall ISO of the film decreases.

How to Address Reciprocity Failure

Now the question is, of course, how do you know how much exactly to compensate for reciprocity failure? How do you know how much additional time to keep the shutter open compared to what your meter suggests? Thankfully, there’s no rocket science involved.

Almost every film type experiences reciprocity failure differently, so manufacturers are kind enough to include tables and references of recommended settings on the packaging. When in doubt, check the official website.

In cases of uncommon or low-volume film rolls, which may not have as much helpful documentation available, there are also plenty of mobile apps you can download to calculate reciprocity failure automatically.

Simply use readings from a light meter as a reference and plug in the corresponding values. Finding the correct exposure is then just a matter of switching to manual mode and making the right adjustments to the shutter speed.

This case of reciprocity failure only applies to film users. Digital cameras are immune from the loss in light sensitivity as there is no chemical reaction to worry about. Because of this, reciprocity failure is largely treated as a film-only phenomenon that digital shooters need not worry about.

Utilizing Reciprocity to Find the Right Aperture and Shutter Speed

Used effectively, reciprocity can be one of the most powerful tools you can have as a photographer. It works with every kind of camera and gear, and it can streamline the entire process of determining the right exposure without relying on external tools.

The best thing you can do while you’re still learning is to practice reciprocity with the help of a light meter. Begin by setting a manual ISO value as low as possible, given ambient light conditions. Proceed by manually setting the aperture and shutter speed to the values suggested by the meter.

Take a test shot using these settings so you’ll have something to compare to afterward.

Now consider the effects of, say, a faster shutter speed on the final image. Think of what kind of photo you are aiming to capture and how a change in exposure settings could help you get there. Try to achieve this by just manipulating one value first.

Let’s say that you decide to open up to the maximum aperture because you desire the thinnest depth of field that you can get in this shot. Using reciprocity now, without relying on the light meter, try to adjust the shutter speed so that the exposure value (EV) of your shot remains constant.

When you’re confident, take the shot and compare it with the test photo you just took earlier. What stands out? How may you improve? How did your meter help, and where did it fall short?

Use this experience as a guideline to improve your skills and judgment. With time, you will find that you’re able to use reciprocity as an intuitive tool and that it will complement your photographic workflow quite naturally.

Good luck, and have fun shooting on the way there!

Take Away

Reciprocity is a complex topic and one that doesn’t just take a solid grounding in theory, but also many hours of practice to master. Thankfully, having read this guide, most of the fundamental theoretical aspects are behind you now. Don’t despair if a lot of what we covered today does not sink in immediately! Instead, try to focus on the most important aspects of exercise: Internalizing the crucial techniques, such as correlating your aperture and shutter speeds, and practicing that pattern as much as you possibly can.

Try to do some math in your head while you’re composing, even if it is a little difficult, to understand how changes on one end of the exposure spectrum affect the look of your image and how you can compensate for that without using your light meter for help. Soon enough, the principles behind reciprocity will undoubtedly sink in, and you will be rewarded with images that make you proud!