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Equivalent Focal length – What does it mean?

Equivalent Focal Length

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 a differently sized sensor. For instance, the Micro Four Thirds equivalent focal length to a 50mm full-frame lens is 25mm.

Getting to grips with equivalent focal length can be a little 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


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Table of Contents

What is Equivalent Focal Length

Focal length and equivalent focal length are used to determine your lens’s angle of view.

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. Whilst 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 – the first being focal length.

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 Guide to Focal Length.

To illustrate, the chart below presents the focal lengths and angle-of-views for lenses placed on 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 is capturing twice the angle of view.

The illustration below shows how different sensors capture images relative to their own size. In other words, a large sensor captures a larger image than a smaller sensor.

As you can see. The larger full-frame sensor is capturing a larger image, thus providing a wider angle of view. Whereas, the smaller Micro Four Thirds sensor captures smaller images and therefore, a narrower 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?

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 with 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 simply 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 if you are working with different systems, the sensor sizes and crop factors are different. 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.

To make things a little easier, I have included all popular crop factors in the chart below. The download includes more, such as crop factors for Medium Format and 1-inch sensors.

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. Well, 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 do a 35mm lens which should be close enough.

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Conclusion

A 50mm lens on one system can be very different 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 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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