lovelifey: [Photography] The Reciprocal Rule in Photography, Explained

The post The Reciprocal Rule in Photography, Explained appeared first on Digital Photography School. It was authored by Adam Welch.

a guide to the reciprocal rule in photography

If you’re looking to capture sharp photos, then the reciprocal rule is an excellent starting point.

But what is the reciprocal rule? How does it work? And how is it affected by crop factors and image stabilization?

In this article, I explain everything you need to know about the reciprocal rule in photography. By the time you’re done, you’ll know how to apply it to your own photos for consistently sharp shots.

Let’s get started.

What is the reciprocal rule in photography?

The reciprocal rule states that to capture sharp photos, your shutter speed should be at least “1” over your lens focal length. In other words, to keep your shots sharp, you should use a shutter speed that is the reciprocal of your lens. Otherwise, you’re at risk of blur due to camera shake.

So if you use a 50mm lens, you’ll need a shutter speed of at least 1/50s. And if you use a 200mm lens, you’ll need a shutter speed of at least 1/200s. Make sense?

The reciprocal rule comes with several qualifications:

It applies to full-frame cameras, not APS-C cameras (more on this later!)
You must be shooting with a relatively steady base and in low wind
It’s designed purely for handholding; if you’re using a tripod, you can work at far slower shutter speeds than the reciprocal rule suggests

Note that the reciprocal rule, despite its name, is not really a rule. It’s a guideline, an easy way to stop handheld-induced camera shake from ruining your photos. There are certain situations – as I discuss below! – when you can ignore the reciprocal rule, or you can adjust the reciprocal rule to get good results.

Why does the reciprocal rule work?

The reciprocal rule is all about preventing handheld camera shake from ruining your images. You see, when you work handheld, camera shake is a fact of life – but use a fast-enough shutter speed, and any camera shake will be negated by your split-second shutter.

But why does focal length make a difference? Why doesn’t every lens have the same “fast-enough” shutter speed?

Longer focal lengths have a more constrained field of view. In other words, longer focal lengths magnify the world, thereby amplifying camera shake.

So when you shoot at 50mm, a little camera shake isn’t such a big deal. But zoom out to 600mm, and even the slightest movement becomes a problem, which means you must increase your shutter speed to compensate.

If you’re struggling to understand this concept, then check out my video, where I explain why the reciprocal rule works (using visuals!):

Bottom line: The goal of the reciprocal rule is to increase your shutter speed as the focal length gets longer, thereby counteracting magnified camera shake.

However, the reciprocal rule does depend on certain conditions, which I dive into below:

The reciprocal rule and body stability

The reciprocal rule assumes your body is relatively stable; it also assumes that you’re using decent camera technique. So it doesn’t apply if you’re standing on one leg, jumping up and down, or otherwise imbalanced.

Instead, for the reciprocal rule to work, you should be standing with your knees slightly bent, both hands holding your camera, and your elbows tucked in close to your side or chest. This can be a problem when using your camera’s LCD screen to compose – your arms go out, which destabilizes the camera.

External factors can affect body and camera stability, too. If you’re shivering due to the cold, or you’re being blown this way and that due to high winds, then the reciprocal rule will give you a too-slow shutter speed.

Therefore, I recommend you increase your shutter speed past the reciprocal rule if you’re:

Imbalanced
Working in wind
Holding your camera away from your face

It’s impossible to give an exact recommendation for these scenarios, so I’d suggest you do a few tests. Take some photos while using the LCD screen, take some photos while poorly balanced, and so on. Work at different shutter speeds and check the results. Then develop a modified reciprocal rule for each scenario.

Happily, certain scenarios allow you to relax the reciprocal rule. If you’re able to gain extra stability by leaning against a solid surface (such as a tree or a brick wall), then you can drop the shutter speed past the reciprocal rule’s “allowed” value. You can also drop your shutter speed when shooting from a kneeling position, and you can drop it even further when lying on the ground.

Sensor size and the reciprocal rule

As stated above, the reciprocal rule only applies to full-frame sensors. Smaller sensors, such as APS-C and Four Thirds sensors, crop the focal length (the same way that you can crop a photo in post-processing).

And this magnifies the image.

When discussing APS-C and Four Thirds cameras, you’ll often hear about crop factors. These allow you to determine your lens’s effective focal length or focal length equivalent; you just multiply the actual focal length by the crop factor.

Hasselblad medium format digital sensor and full frame Sony A7r sensor

So if your camera has a 2x crop factor and you’re shooting with a 100mm lens, you simply multiply 100 by 2 for a 200mm effective focal length.

And it’s your effective focal length that you should use with the reciprocal rule.

In other words, to apply the reciprocal rule to cropped sensors, you must first determine the effective focal length, then calculate your shutter speed minimum via the reciprocal rule.

If you’re not certain of your camera’s crop factor, you can always do a quick online search, but here are some common factors:

Canon APS-C: 1.6x
Nikon APS-C: 1.5x
Fujifilm APS-C: 1.5x
Four Thirds/Micro Four Thirds: 2x

Also, while I don’t want to make things too complicated, know that you must do the same when using sensor sizes larger than the full-frame standard – but you’ll use a reverse crop factor (such as 0.8x, 0.5x, etc.). Medium- and large-format cameras feature large fields of view, increasing the duration of your allowable handheld shutter speed.

The effects of image stabilization

Some cameras and lenses offer image stabilization, which helps counteract handheld camera shake.

As you can probably guess, this means that you can use a slower shutter speed than recommended by the reciprocal rule – but how slow can you go?

Image-stabilized cameras and lenses come with a reported stabilization value – generally in the area of three to five stops – and you can use this to calculate a modified reciprocal rule.

(Stops are a way to talk about changes in exposure variables; a single stop corresponds to a doubling or a halving of your shutter speed value.)

Simply identify your reciprocal rule shutter speed, then reduce it by the indicated number of stops.

That said, if you want an even more accurate guideline, you should test each lens and camera. Take a series of shots while reducing the shutter speed. See how low you can take the shutter speed while still producing sharp photos, then commit the number to memory.

When the reciprocal rule fails

The reciprocal rule is a useful method for calculating shutter speed minimums. And as you’ve seen, you can modify the reciprocal rule to deal with crop-sensor cameras, image stabilization, and more.

But there are a couple of situations in which the reciprocal rule fails completely – so completely, in fact, that you’ll need to ditch the rule and think in different terms.

Shooting with a tripod

A good tripod stabilizes your camera so completely that you can drop your shutter speed as much as you want and you’ll still capture a sharp shot.

In other words, if you’re working with a tripod, you can forget about camera shake, you can forget about the reciprocal rule, and you can just lengthen your shutter speed until you get the result you’re after.

Of course, you must be using a solid tripod and it must be set up on stable ground. You’ll also need to use a two-second self-timer or a remote release to prevent mechanical camera shake.

But as long as you use proper technique and a sturdy tripod, you’ll capture crisp images no matter your shutter speed.

Shooting action

The reciprocal rule is designed to prevent blur due to camera shake. Unfortunately, that does not extend to blur caused by moving subjects, such as people walking, cars racing, or birds flying.

Which means that, when you’re capturing action, you’ll need to use a shutter speed far higher than the reciprocal rule suggests.

The specifics depend on the subject, but I’d recommend starting at 1/250s for slow-moving subjects, 1/1000s for fast-moving subjects, and 1/2000s for very fast subjects (such as birds in flight). Note that you’ll still need to increase your shutter speed as you increase your lens’s focal length, but the reciprocal rule itself will be largely useless.