How is focal length different from flange focal distance?
Asked 9/2/2013
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I’m confused about the difference between focal length and flange focal distance. I thought focal length was the distance from the rear of the lens to the sensor, but that doesn’t seem to fit lenses for different mounts.
For example, an EF-M lens can sit closer to the sensor than an EF-S lens, yet both can have the same focal length. If that’s true, what exactly is focal length measured from, and how can lenses with different flange distances have the same focal length?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Focal length is not the distance from the rear element to the sensor, it's the distance of the nodal point of the objective lens to the plane of focus. In a simple system, the lens may have only one glass element and that's the real distance, but in a complex lens, the various elements are formed to make the physical lens much smaller (or sometimes longer for very short focal lengths) than the actual focal length and to work within the flange focal distance, so you can't physically see the focal length.
If you think about it for the moment, it's fairly obvious. Somebody with an 800mm lens does not have the rear element almost a meter from the camera sensor, the lens would be utterly monstrous (they are now, this would be much worse). So, there are obviously more complex systems at play here as a result.
Originally by user472. Source · Licensed CC BY-SA 4.0
user472
12y ago
0
Generated from our catalog & community — verify before relying on it.
Focal length is not the distance from the rear element of the lens to the sensor.
For a lens focused at infinity, focal length is measured from the lens’s optical center (more precisely, the rear nodal point) to the image plane/sensor. In a simple single-element lens, that may roughly match the physical spacing. But modern lenses use multiple elements, so the optical center can be located somewhere inside the lens system rather than at the rear glass.
That’s why physical lens length, rear-element position, and mount flange distance can all differ from the stated focal length.
Flange focal distance is a separate mechanical specification: the distance from the camera mount flange to the sensor. Different camera mounts can have different flange distances, and lens designers compensate optically so the lens still produces the same focal length and focuses correctly.
So an EF-M lens and an EF-S lens can both be, say, 18mm lenses even though one mount places the lens closer to the sensor. The optics are simply arranged differently to achieve the same effective focal length.
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