I'd like to know how to calculate the new focal length when using a lens adapter. Is there any formula?

To be more specific: I need to attach my old lens (four third) to new micro four third system with MMF-2 adapter.


Lens mount adapters increase the pool of available lenses we are able to mount on any particular camera body. Adapters are basically mechanical coupling devices; however, some also couple electrical communications as well. Unless the adapter incorporates optics, the optical properties of the adapted lens are not altered. In other words, the focal length of the adapted lens does not change.

  • Isn't the f length is the distance between the optic and the focus point? Isn't adding an adapter increases this distance?
    – inckka
    Mar 28 '19 at 7:01
  • 1
    @inckka no, the adapter give you the correct distance, otherwise you wouldn’t be able to get sharp images. That is why you can mount a lens with long flange distance to a body that has a low distance using a man adapter - but you cannot go the other way around. Eg you will not be able to mount a Canon lens on a Nikon body, but you can do it the other way around.
    – ssn
    Mar 28 '19 at 7:48
  • The focal length is a measurement taken between lens and focused image when the lens is imaging a far distant object (like measuring lens to paper distance when using a lens as a burning glass focusing the image of the sun on paper). The camera lens is complex array of lenses. Where is the point on the lens we measure from? A lens has two principle points. We find these on an optical bench and use the rear point (rear nodal) and find distance to focused image when the subject is at infinity. Mar 28 '19 at 15:38
  • What about the FOV? is it getting changed when used an adapter?
    – inckka
    Mar 29 '19 at 8:57

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