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How to Find the Effective Focal Length of a Lens

If you shoot with a 50mm lens, it’s focal length is 50mm, right?

Well, that depends…

On a full frame camera - one with a sensor that has a crop factor of 1:1, a 50mm lens does indeed perform like a 50mm lens.

But many cameras these days have a sensor that is smaller than a full frame, which means their crop factor might be 1.5x, 1.6x, 2x, or various others.

What does all that mean for the effective focal length of your lens?

 

Terms You Need to Know

 

 

First, let’s get some terminology out of the way.

Crop Factor is the ratio of a camera’s sensor size relative to a 35mm full frame sensor. That means that if your camera has a crop factor of 1.5x, a full frame sensor is 1.5 times larger than the sensor in your camera.

Focal length refers to the distance from the imaging sensor to the point in the lens at which light rays converge. Since that’s a little technical, most people simply utilize focal length to understand how much of a scene will be captured by a lens. For example, a very short focal length like 18mm will capture a very wide view of a scene, as shown in the image above. A very long focal length like 300mm will capture a very narrow view of the scene, as shown below.

 

 

Effective focal length is essentially a combination of the two previous terms. The crop factor of the camera must be considered because it changes the effective focal length of the lens. In other words, a 50mm lens on a camera with a 1.5x crop factor has an effective focal length of 75mm.

 

Calculating the Effective Focal Length

 

 

As noted above, calculating the effective focal length of a lens used with a crop sensor camera requires that you know the crop factor of the camera you’re using.

Canon crop sensor cameras have a crop factor of 1.6x, as do FujiFilm crop sensor cameras. Nikon, on the other hand, has a crop factor of 1.5x while many Olympus crop sensor cameras have a 2x crop factor.

That means that you need to identify the crop factor of your particular camera first, and then multiply it by the focal length of the lens you’re using to identify the effective focal length.

For example, if you’re shooting with an 85mm lens on a Nikon crop sensor camera, you’d multiply 85 x 1.5, which is 127.5. As another example, if you’re using a 24mm lens on an Olympus crop sensor camera, you’d multiply 24 x 2, which is 48.

So, while the term effective focal length can cause a lot of confusion, figuring out the effective focal length of a lens is actually quite simple.

 

What Effective Focal Length Means For Your Photos

 

 

Obviously, if you shoot with a crop sensor camera, the effective focal length of the lens you use can have a major impact on what your image looks like.

For example, if you’re using a 35mm lens on a Canon crop sensor camera, your photos won’t have the “standard” or “normal” angle of view that a 35mm lens produces on a full frame camera.

Instead, with the 1.6x crop factor taken into account, that 35mm lens will behave more like a 56mm lens with a much narrower field of view of the subject.