Equivalent Focal length refers to the focal length required by one camera sensor size to match the angle of view of a focal length being used by another. For instance, the Micro Four Thirds equivalent focal length to a 50mm full-frame lens is 25mm.
Understanding equivalent focal lengths can be tricky. But once you’re up to speed, you’ll be better able to choose the right lens and camera system to meet your photography needs. So, let’s begin. Jump to Conclusion
Table of Contents
What is Equivalent Focal Length and Angle of View
Focal and equivalent focal lengths are used to determine your lens’s angle of view. Angle-of-view is measured in degrees and represents how zoomed-in or out you are.
For example, lenses with a wide angle of view are great for photographing larger scenes such as landscapes. At the same time, lenses with a narrow-angle of view are better for zooming up close and taking photos of distant subjects.
In practical terms, your lens’s angle of view is determined by two factors – focal length and sensor size.
Focal Length
Your lens’s focal length is measured in millimeters, with shorter focal lengths, such as 28mm, producing a wider angle of view than longer focal lengths, such as 200mm. Read What is Focal Length in Photography.
To illustrate, the chart below presents the focal lengths and angle-of-views for lenses mounted to a full-frame camera.
Sensor Size
The second factor determining your angle of view is sensor size. Sensors come in many different sizes, with larger and wider sensors capturing a wider angle of view than smaller sensors. Read Beginners Guide to Sensors.
Therefore, if you put a 210mm lens in front of a full-frame sensor and an identical 210mm lens in front of a smaller Micro Four-Thirds sensor – you will end up with two entirely different angle-of-views. This is because the full-frame sensor is twice as large and, thus, captures twice the angle of view.
The illustration below shows how different sensors capture images relative to their size. In other words, a large sensor captures a larger image than a smaller sensor.
As you can see. The larger full-frame sensor captures a larger image, thus providing a wider angle of view. In contrast, the smaller Micro Four Thirds sensor captures smaller pictures and, therefore, a more narrow angle of view.
But what if you want your Micro Four Thirds camera to capture the same angle of view as the 210mm lens on the full-frame camera? Well, you need to equip your Micro Four Thirds camera with a lens featuring a focal length equivalent to 210mm on the full-frame camera. Read Is Full-Frame worth it?
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Equivalent Focal Length
Equivalent focal length is the focal length required by one sensor size to replicate the same angle of view as a focal length used by a differently sized sensor.
For example, to get the same 47-degree angle of view as a 50mm lens on a full-frame lens – you would equip your Micro Four Thirds camera with a 25mm lens. Therefore, 25mm is Micro Four Thirds’ equivalent focal length to a 50mm lens on a full-frame camera.
How to calculate Equivalent Focal length using Crop Factor
Crop-factor is the difference between one sensor’s diagonal length and another. For instance, a Micro Four Thirds sensor’s diagonal length is roughly half that of a full-frame sensor. As a result, the crop factor is one-half or 0.5.
Therefore, to find the Micro Four Thirds (M43) equivalent focal length to a 50mm lens (or any other) on a full-frame camera, you multiply 50 by 0.5, revealing an equivalent focal length of 25mm.
Furthermore, the same process works in reverse. If you have a 25mm M43 lens and wish to find out its full-frame equivalent, simply double 25mm to determine its full-frame equivalent of 50mm.
But the sensor sizes and crop factors are different if you are working with different systems. For instance, the crop factor used to calculate an equivalent focal length from M43 to APS-C is 1.31.
Therefore, to find the APS-C equivalent focal length to a 25mm lens on Micro Four Thirds, we multiply 25mm by 1.31 – revealing an equivalent focal length of 32mm.
I have included all prevalent crop factors in the chart below to simplify things. The download consists of more, such as crop factors for Medium Format and 1-inch sensors.
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Crop Factor Chart
Download Crop Factor Chart
Free Download. Includes all common sensor sizes
Best focal lengths by use case
But what if you don’t want to waste your life calculating crop factors? The table below shows the ideal focal length for each system by use case.
In reality, you might not be able to get an exact match for your system. For instance, Nikon does not make a 33mm lens. However, it does a 35mm lens which might be close enough.
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Conclusion
A 50mm lens on one system can behave very differently from a 50mm lens on another. While getting to grips with equivalent focal length requires some mental gymnastics, it soon becomes intuitive as you settle down into your chosen camera system.
And when you do grasp an equivalent focal length, you will be better able to choose the lenses and camera system that best suits your photography needs. Back to Intro
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