Hallöchen!
Fabien Castan writes:
> [...]
>
> The focus has an impact on the focal length. I guess that the
> metadata from the camera doesn't take the focus into account.
At least for Nikon, it doesn't. Probably neither for any other
vendor.
> On the camera device, it's just a mechanical record of the focal
> length choose by the user on the device. The theoretical focal
> length value matches the real focal length of the lens/camera
> device when the focus is at infinity. So if you use the focal
> length from the metadata and the focus is not at infinity, you are
> using a wrong focal length.
In general, yes. With the <real-focal-length> tag, Lensfun keeps
track of nominal vs. actual focal length, e.g.:
<real-focal-length focal="8" real-focal="8.405" />
However in most cases, it is not worth it. The benefit is largely
theoretical.
> If we use a checkerboard for calibration, we compute both the
> focal length and the distortion, so the result will be correct
> because this relationship is independent from the focus.
If I understand you correctly, you assume that a lens at 200mm
nominal and close focus, which actually has only 135mm, exhibits the
same distortion as the same length as 135mm nominal and focus at
infinity?
> [...]
>
> Are you interested if we propose a pull request with a new calibration
> binary (with OpenCV optional at compile time)?
I personally no. I do not believe in checkerboard calibrations for
reasons I have stated earlier.
Tschö,
Torsten.
--
Torsten Bronger Jabber ID: torsten.bron...@jabber.rwth-aachen.de
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