The reciprocal rule in photography states that your shutter speed should be at least as fast as the reciprocal of your focal length (1/focal length).
Meaning if you have a 50mm lens you need a shutter speed of 1/50th of a second and if you have a focal length of 100mm you need a shutter speed of 1/100th of a second.
You can use this rule to reduce that dreaded handheld camera shake, so that you can have sharper photos.
Some key takeaways from the reciprocal rule:
- The wider/shorter the focal length, the slower you can set your shutter speed without risking camera shake.
- With narrower/longer focal lengths, you need faster shutter speeds to avoid camera shake in your photos.
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How Crop Factor Affects The Reciprocal Rule
Since the reciprocal rule is dependent on the full frame equivalent focal length of a camera, you have to think about the crop factor of your camera (if it has one).
Your camera will have a crop factor if it is a crop sensor camera and not a full frame DSLR or mirrorless camera.
If you do have a crop sensor camera your shutter speed will have to be faster than if you are using a full frame camera with the same lens.
💡 Let’s look at an example of using the Canon EOS Rebel SL3 / 250D which is a crop sensor Canon camera with a crop factor of 1.6x.
A 50mm lens on this crop sensor Canon camera needs a shutter speed of 1/80 to follow the reciprocal rule in photography.
This is because the full frame equivalent focal length on this camera would be 80mm.
On the other hand if you are using a 50mm lens on a full frame Canon camera then the shutter speed will only need to be 1/50.
If you’re wondering if your camera is a crop sensor camera or wanting to know the crop factor of it then do a quick Google search as follows:
- “Camera name” crop factor
- Is “camera name” crop sensor
You should be able find the crop factor of your camera pretty easily by doing this.
- Canon crop sensor cameras have a crop factor of around 1.6x
- Nikon, Sony, Fuji, Pentax crop sensor cameras have a crop sensor of around 1.5x
Why Do We Use The Reciprocal Rule?
When you’re shooting handheld with a camera there’s a natural amount of camera shake that you’ll get because, well, you’re a human and not a tripod.
The reciprocal rule is a general rule of thumb that tries to find out the minimum shutter speed needed for different focal lengths, to make sure that you don’t see this camera shake in your photos.
You don’t need to use the reciprocal rule if you are using a tripod because there shouldn’t really be any camera shake (unless you’re holding the tripod in the air like a lunatic).
Step by Step : How to Use the Reciprocal Rule
You can’t just learn by reading in photography.
You’ve gotta take action.
So follow these steps below with your DSLR or mirrorless camera to see the reciprocal rule in action.
- First find out if your camera is a crop sensor or full frame. (Hint: if you’re asking what the reciprocal rule is you probably have a crop sensor camera.)
- In this example we will use a Canon Rebel SL3 / 250D which has a crop factor of about 1.6x.
- Once you’ve done that, choose a lens to attach. We’ll assume a 50mm lens in this example.
- Now work out your full frame equivalent focal length if it is a crop sensor camera.
- On a Canon Rebel SL3 the full frame equivalent focal length of a 50mm lens is about 80mm because of the crop factor of 1.6x.
- Now work out the shutter speed value needed using the reciprocal rule.
- On the Canon Rebel SL3 it would be 1/80th of a second due to the full frame equivalent focal length being 80mm.
- Now take a picture of some text in a book using the shutter speed of 1/80, not too close and not too far away. About an arm’s length or so will do.
- Now take another picture with a shutter speed half as fast but from the same spot and distance. So if you used 1/80th of a second. Now use 1/40th of a second.
- Just for good measure try one with a shutter speed faster than your original. e.g. if you used 1/80 try using 1/160.
- Look at the results and compare the differences.
- What did you see?
- Was there a difference between the photos?
You should see that using the shutter speed of 1/80 or 1/160 in this scenario should give you a sharper image with little to no camera shake.
While on the other hand, the image using the shutter speed of 1/40 won’t be as sharp because it doesn’t meet the minimum shutter speed requirement of the reciprocal rule in our example.
It’s always important to put theory into practice because results can vary.
Maybe you have steadier hands than the average person.
Or maybe you’re really shaky, so you need to use a faster shutter speed than the reciprocal rule suggests to make sure your images aren’t a mess.
Whatever the case, I’m sure this tip will help you as a useful guide for your shutter speed for handheld shots.
When the Reciprocal Rule Won’t Work
The reciprocal rule is mainly useful for a still or very slow moving subject.
If you have a moving object like a car then the reciprocal rule won’t really help.
You’ll avoid camera shake, but you won’t eliminate motion blur because that’s a different thing altogether.
Don’t think that the reciprocal rule is the only thing to consider when choosing your shutter speed or you might be bashing your head against a wall thinking “Why is this so blurry?!”.
After all, the reciprocal rule just suggests the minimum shutter speed that you should use to avoid handheld camera shake.
If your subject is a fast moving car or sports-person then you’ll most likely need a shutter speed faster than the reciprocal rule would suggest.
In these kinds of situations, if you want to freeze the subject in frame then you’ll need a higher shutter speed.
But if you are at the limits with available light, then you might need to open your aperture to a wider f-stop or bump your ISO up higher.
This would allow your camera to let more light in and still expose correctly while increasing the shutter speed.
Or you could even use artificial light like an external flash if you have access to that.
Although if you’re out on a bright sunny day you should be fine.