In one of my previous posts I mentioned the “focal length reciprocal rule” and it occurred to me that not all of my readers may know what that is. Rather than find someone else's article about it (of which I'm sure there are many) and link to it, I thought I'd just write my own.

The goal of the rule (which is more of a guideline than a rule, actually) is to give you an idea of whether a photograph will come out blurry if you're holding the camera in your hand. It is a somewhat simple formula to determine how various camera settings combine to compensate for camera shake. I use the rule constantly while I'm out shooting because I don't like to bring a tripod to most places and there's no use bringing home a whole CF card filled with blurry photographs.

Not only will I explain what this reciprocal rule is, but I'll give you real examples of how to use it in the field.

The Reciprocal Rule

In its simplest terms, the rule states that you can safely handhold the camera if your shutter speed is faster than the reciprocal of your effective focal length. That may sound slightly daunting–and I guess that's why people write articles about it–but it's not a very hard thing to figure out in your head once you get the hang of it.

So first of all, what is a reciprocal? Everyone probably learned this in high school math, but as I was always quick to point out right before exams, who uses that stuff, anyway? A reciprocal is what you get when you flip a fraction upside down. Because every whole number is a fraction made up of itself over one (3 is the same as 3/1), the reciprocal of a whole number is one over that number. For example, the reciprocal of 60 is 1/60.

Before I get further into the math, let me start by explaining how I use the reciprocal rule. My preference is to stay in Av or Aperture Value mode basically all the time, so I choose an aperture and my camera will meter the scene and tell me what shutter speed it thinks I should be using to get a proper exposure. By manipulating my ISO sensitivity, aperture setting, and EV (Exposure Value), I can cause the metered shutter speed to become faster or slower.

The goal (while using this rule) is to manipulate the values you have control over until the shutter speed is faster than the reciprocal of your focal length. When figuring out your focal length remember these important caveats:

  • If you are using a zoom lens, your focal length depends on how far you are zoomed in. With a 24-70mm lens for example, your focal length could be anywhere between 24mm and 70mm. You can generally use the markings on the barrel of the lens to ballpark the current focal length; it doesn't have to be perfect. If you want to round it to the nearest actual marking, round up to be safe.

  • If you are using a digital camera with a sensor smaller than full-frame (e.g. APS-C, etc.), which includes the Canon 10D, 20D, 30D, all of the Rebels, all Nikons, and basically every point-and-shoot camera on planet Earth, you need to multiply the current focal length of your lens by your camera's multiplier to get the true effective focal length. As an example, the 10D has a 1.6x multiplier, so a 24mm lens on the 10D has an effective focal length of 24mm x 1.6 = 38.4mm. If you don't know what your camera's multiplier is, consult the manual, Digital Photography Review, or search Google.

I know I just made this “easy” guideline sound even more complicated, but as I mentioned before, it's just a guideline. It might be easier to multiply by 1.5 and then just assume the answer is too slow by one stop, or take your best guess. There is no substitute for experience in this case.

Example

Here is a “real-world” example that will show you how the numbers are put together to give you an answer. I simulated these settings with my camera, so you can be sure that they're pretty close, but as always, YMMV (Your Mileage May Vary).

I am working with my 50mm f/1.4 lens on the 5D (which is full-frame), so my effective focal length is always 50mm. That should make it a bit easier to explain.

Using Av, or Aperture Value mode, I set my aperture to f/1.4. My ISO is 100. I meter the scene and the camera tells me I need a shutter speed of 1/8 second to get the proper exposure. 1/8 is much larger than the reciprocal of my focal length (which is 1/50, right?) so I need a faster shutter speed.

Opening the aperture would give me more light, but it's already as far open as it can go. I have two choices:

  • Add more light to the scene.
  • Increase my ISO sensitivity.

In most cases it's not possible to add more light to the scene when you're shooting in the field, so my only choice is to increase the ISO sensitivity. I set it to 800. Again I meter the scene. Now my camera tells me I need a shutter speed of 1/60. 1/60 is faster than 1/50, so according to the reciprocal rule, it should be OK to expose this photo.

Bear in mind the following caveats:

  • If your subject is moving, the reciprocal rule doesn't help you too much. While it does give a pretty good estimate of the shutter speed necessary to counteract the shake of your hands while holding the camera, it doesn't tell you anything about the shutter speed necessary to freeze any given moving subject.

  • People with a lot of dexterity have been able to take incredibly crisp images of moving subjects at long focal lengths without tripods simply because they are like photographic snipers. I've never had good luck with that.

Image Stabilization (IS) and Vibration Reduction (VR)

Thanks to tukangmoto for reminding me about these features!

The two major camera manufacturers, Canon and Nikon, produce lenses with technology designed to help you with the specific problem of camera shake, which they call IS and VR, respectively. Other camera manufacturers, such as Olympus, Minolta, and Sony, have built this feature into their camera bodies as well.

I don't know the specifics of the system outside of the Canon world, but Canon claims that using one of their IS lenses will allow your shutter speed to be two stops slower than this rule would tell you. When we say “stops,” we basically mean two notches down on your camera's scale. With their latest generation IS, Canon claims that three stops should be possible, but as always, it's better to play it safe, especially when using this rule, because it's an estimate.

If you are using an IS- or VR-equipped camera or lens (with that feature turned on, of course!), you should be safe to achieve a shutter speed one stop slower than the rule tells you to. In other words, if the rule estimates 1/125th is fast enough, you can probably get by at 1/60th (each full stop in the shutter scale is about half as long or twice as long, depending on which direction you're moving in; see the Tedious Explanation of the f/stop.)

If you liked this article or have suggestions that might make it easier to understand, leave a comment! Happy shooting.