What's the difference between a drone's true focal length and the 35mm equivalent?
Updated: Nov 30, 2022
It's 2022, and commercial drones over the last 10 years have evolved to carry a number of different payloads. Most commonly used payloads being high definition cameras.
Most of these cameras are unique in their properties which determine the quality of the image captured by the camera. One such property is the drone camera's focal length.
What exactly is focal length and how does it impact drone operations? Also, what is the difference between the true focal length and the 35mm equivalent? We'll explore these questions in the post below.
What is focal length?
Technically speaking, the Focal length of a camera is the measure of how strongly the camera can converge light entering the camera.
Practically speaking, focal length can be thought of as the angle of view of a lens. That is, how much of the target scene does the lens capture and how large do objects appear within the frame?
The longer the focal length of a lens, the narrower its angle of view. Objects appear larger using long focal length lenses than they do viewing them with our eyes. On the other hand, lenses with short focal lengths take in a much wider angle of view. Thus, elements seem much smaller in the frame than they do to our eye.
What is the 35mm equivalent focal length?
But if we have a measure for a camera's true focal length then what is the 35mm equivalent and where did it come from?
In the old days, photos were developed on film which had an imaging area of 35mm. This imaging area is analogous to the modern-day sensor width (diagonal width) used in digital cameras.
However, modern cameras are no longer constrained to using 35mm as the sensor width size. They now come in other sizes - 25mm, 20mm, 15mm and so on. Changing this sensor size changes the camera's true focal length. However, for legacy reasons, focal lengths are often converted into their 35mm equivalent i.e. the effective focal length of the camera when the sensor size is transformed to its 35mm equivalent.
How to calculate True Focal length?
To calculate the true focal length of a lens, from its 35mm equivalent, all you have to do is to divide the 35mm equivalent focal length by its crop factor. The crop factor is the ratio of the sensor's width to a 35mm sensor width. Alternatively, you can divide the diagonal sensor width of the a full frame sensor by the diagonal sensor width of the target sensor.
Crop Factor = 35(mm) / Sensor Width
For e.g. for a camera with a 25mm sensor width, the crop factor will be:
1.5 = 35mm / 25mm
As mentioned above, the true focal length is the 35mm equivalent divided by Crop Factor
True Focal Length = 35mm equivalent Focal Length / Crop Factor
Assuming this camera has a 50mm equivalent focal length, the true focal length will be:
33.3mm = 50mm / 1.5
Want to calculate this for yourself? Get your free and comprehensive spreadsheet designed with Drone Mapping and Inspection formulas over here
How does focal length impact drone operations?
Choosing the correct camera for your drone mission is extremely important. If you are capturing images of a tall far away structure, a large focal length (but narrow field of view) would be the way to go. On the other hand, if you are capturing large and wide close up structures, a shorter focal length should do the trick. As is the case with most drone operations, it's important to use the right tool for the right job. Some DJI drones such as the DJI Matrice allow you to attach variable payloads so that you can use the same drone for multiple jobs. Likewise, Hammer Missions allows you to pick variable drone cameras for mission planning.
Where to use focal length?
It’s important to know whether we are talking about the true focal length of the camera or the 35mm equivalent. For example, when purchasing most DJI cameras and associated lenses, the quoted focal length is often the 35mm equivalent. However, when using this focal length for technical calculations, for instance, calculating GSD (ground sampling distance) values for your drone flight, you should be using the true focal length. You can learn more about GSD calculations here.
List of supported cameras in Hammer Missions
If you're considering using a specific camera with a specific focal length, it is also important to consider whether your drone mission planning tool supports that camera and focal length for planning your flight. In Hammer Missions, we support a wide range of cameras with variables focal lenses, and even allow you to define your camera from scratch! Here is a list of the currently supported cameras in Hammer Missions.
We hope this guide helps you understand your drone's focal length better and it's 35mm equivalent better. If you'd like to learn more about how to high quality data and the get the most out of your drone missions, please feel free to visit our learning resources.
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— The Hammer Team