Understanding the Reciprocal Rule in Photography to Take Better Photos
Discover what the reciprocal rule is, how to use it when determining your shutter speed, and how it is affected by IS, sensor size, and more. This is a great little rule for beginners and a good reminder for advanced shooters, too!
Blur can play a powerful role in photography. It can convey a sense of action and energy. It can isolate a subject and draw your attention to a specific part of an image. The same scene can appear remarkably different due to the presence or absence of blur. But for blur to be effective, it has to be intentional and controlled. Out of control blur can ruin an image.
One type of blur that almost always destroys an image is having the entire shot be blurry because of camera shake. Fortunately, the reciprocal rule in photography can help you avoid missing that otherwise perfect shot.
What is the Reciprocal Rule in Photography?
The reciprocal rule is a rule of thumb for determining your shutter speed. Put simply, in order to avoid camera shake you want to keep your shutter speed at least one over the focal length you are shooting at. So if you are shooting with a 50mm lens, keep your shutter speed at least 1/50th of a second (usually you’ll need to set your dial to 1/60). Using a 200mm lens? Keep it at 1/200th seconds.
Of course, as with all rules of thumb, this is more of a guideline than a rule, and there will be exceptions and times where it doesn’t apply. Let’s look more at camera shake, how to avoid it, and the nuances of the reciprocal rule.
What is Camera Shake?
Any time something changes its relative position to the sensor while the shutter is open, the camera records every position that it’s in, causing it to essentially smear (i.e., blur) together. But what happens when the camera moves while the shutter is open? Relative to the camera, everything in the scene appears to move. Thus, everything in the scene is smeared around a little bit. What you get is an image that is consistently and uniformly blurred.
Camera shake usually happens when you are hand holding a camera with a shutter speed that is too slow. If you’re shooting very early or late, for example, the lower light levels make camera shake more likely. No matter how steady you think you are, you’re naturally going to sway a little bit, or your hands will shake a little bit, or you’ll move just a little when you breathe.
To avoid camera shake, you need to make sure your shutter speed is fast enough that these micro movements are not noticeable in the final image, which is where the reciprocal rule comes into play.
Understanding Shutter Speed
As you probably know, shutter speed is how long the shutter in your camera stays open, allowing light to hit the sensor. It’s typically measured in fractions of a second, though longer shutter speeds are measured in full seconds. The slower the shutter speed, the more light the camera lets in, but the more likely moving objects are to blur. The faster the shutter speed, the less light the camera lets in and the more it freezes any motion.
Depending on the subject, you might specifically need a faster or slower shutter speed. For example, if you’re shooting a sports game you’ll likely need a very fast shutter speed to capture a specific moment.
What shutter speed do you need to use to avoid camera shake? Well, there are a few variables that determine that, but perhaps the biggest is the focal length of your lens.
Understanding Focal Length
The focal length of your lens is the distance from where the light converges inside of a lens to where it hits the sensor. The shorter this distance (the shorter the focal length), the wider the field of view will be. Very short focal lengths might have a field of view approaching 180 degrees (that is, you’ll be able to see everything in front of them). Very long focal lengths might have a field of view of just a few degrees.
How does this relate to camera shake? Think back to motion. In a very long lens, your field of view might be five feet from side to side while a very short lens might have a field of view one hundred feet wide. If an object moved a distance of one foot, that’s 20% of the way across the longer lens’ field of view, but only 1% of the way across the shorter lens’. The longer the lens, the more that any motion will be amplified.
Similarly, motion resulting from camera shake is going to be amplified in a longer focal length lens compared to a shorter focal length lens. In order to avoid the blur that camera shake causes, you will need a faster shutter speed for a longer focal length. The reciprocal rule gives us a guideline as to what speed we need to be shooting at in order to avoid camera shake. Shooting at one over the focal length should give you a decent starting point to avoid camera shake.
The Reciprocal Rule in Full Frame vs. Crop Frame Sensors
It is important to keep in mind that the reciprocal rule was developed in the days of film. It works well for cameras shooting on film or on sensors that are the same size (i.e., full frame sensors – or 35mm equivalents). But what about crop frame sensors?
By definition, a focal length when paired with a crop frame sensor is exactly the same as with a full frame sensor. However, because the crop sensor is smaller and only records a fraction of what a full frame sensor records, the field of view is smaller at any given focal length. You often hear people refer to the “effective focal length” to distinguish the effects that the smaller sensor has on the field of view.
For example, Nikon and Sony cameras have a 1.5x crop factor, so a 100mm lens on one of their crop cameras will have the same field of view as a 150mm lens on a full frame camera. The 100mm lens is effectively a 150mm lens when put on that crop camera. For Canon, their crop sensors have a 1.6x crop factor.
When considering the reciprocal rule for crop cameras, you need to calculate based on effective focal lengths. Simply using a 1/50th or so seconds shutter speed on a 50mm lens will likely result in camera shake. To be safe, use 1/125.
A Note on Image Stabilization
In 1995 Canon released what would be a game changer and upend the reciprocal rule: the first commercially-available SLR lens with built-in image stabilization. Since then, every lens manufacturer has developed image stabilization systems for their lenses (though not all lenses incorporate them). Even more recently, cameras themselves have begun to incorporate image stabilization directly into the body of the camera.
Image stabilization systems help to reduce the amount of shake that is transmitted through the lens and onto the sensor. Image stabilization efficiency is generally measured in how many stops you can slow the shutter speed to get the same results.
Most image stabilization systems offer between 2 and 4 stops of stabilization. In other words, for a 200mm lens, instead of needing a shutter speed of 1/200th seconds, you could use as slow as 1/50th (for a 2 stop stabilization) or even 1/15th second (for a 4 stop stabilization). The benefit of this is achieving sharper images in low light without having to crank your camera’s ISO (which can introduce unwanted noise).
Body Position and Blurry Photos
Again, the reciprocal rule is a guideline more than a rule. In addition to whether you have a full frame or crop sensor and whether or not you have image stabilization, there are other factors that will determine whether you’ll have motion blur in your pictures.
The first is simply how stable you are. If you have shaky hands anyway, you might need to bump your shutter speed a little faster than the reciprocal rule would tell you to. Or you might find that you are good at standing more still than average and can go with slightly slower speed before camera shake becomes a problem.
Make sure that you’re holding the camera with a stable grip. The best grip is to gently hold the lens underhand so that the weight of the camera is on your entire hand as opposed to an overhand with just your thumb below the lens.
If you keep your elbow tucked into your side, it can offer an additional level of support for the weight of your camera, allowing your body to hold more of the weight. If you use a camera strap, adjust it so that there’s a little bit of tension when it’s up to your eye. This will limit the number of directions that the camera might drift toward that cause camera shake. Finally, stand in a comfortable and stable posture, with your legs slightly spread. For many people, having one foot slightly in front of the other (staggered stance) provides a little more stability.
How to Apply the Reciprocal Rule While Shooting
Assuming that you have good shooting posture and are in a comfortable position, take a moment to consider the reciprocal rule to check whether or not you need to make adjustments to your shutter speed. Think about what your focal length is. Think about whether you need to apply a crop factor to adjust the effective focal length. With these in mind, come up with your base shutter speed.
Next, think about whether or not your lens or camera has image stabilization. If it does, do you know how many stops it’s rated for? For each stop, you can cut your shutter speed in half. If you don’t know, estimate two stops just to err on the side of caution. This is your new minimum shutter speed.
Is the shutter speed you want to use faster than this minimum shutter speed? Great! If it’s not, figure out how you can make adjustments to avoid camera shake. Maybe you can raise the ISO or use a wider aperture to allow you to speed up your shutter. Or maybe you will have to use a tripod. But at least by knowing that number you can make an educated decision about how to proceed.
Blur in photos can be a very useful technique. But if you can’t control the blur, you are going to ruin a lot of images that could otherwise have been great. Fortunately, the reciprocal rule has been developed to help guide us so that we can avoid camera shake. It’s not a firm or perfect rule, and there are certainly times to ignore it, but if you use it as a way to check your exposure settings you can avoid a lot of disappointment and heartache.