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I work with C-mount cameras a lot and I've noticed that the lens' manufacturers often list back focal length (BFL) among optical specifications. (That's the distance between the lens' last optical element and the sensor.)

Why do they specify that? What is the significance of BFL to me, a customer, who just buys and uses the lens? Isn't that number just an implementation detail? I only care that the lens complies with the C-mount standard, meaning that it'll fit onto the camera and project the image onto the plane where the sensor is located.

For instance, it'd be much more useful if they specified the position of the front principal plane, which affects camera positioning. Or pupil magnification, which affects where the center of perspective is and the light gathering ability & DOF at high magnifications.

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Never mind, found the answer on photography stack exchange: https://photo.stackexchange.com/questions/61728/what-is-the-importance-of-the-back-focal-length-of-a-lens

Basically, it's just a mechanical constraint some people may care about: either there's something inside the camera between the lens and the sensor or they need to know how much space is required to remove the lens.

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