Hi Torsten, I have read some papers but it's still unclear to me why the focus at infinity is so important. It is said in Bown 71: “Radial distortion is normally calibrated at infinity focus” but without explanation.
In Debevec paper: "It should be noted that most prime lenses actually change in focal length depending on the distance at which they are focussed. This means that images taken with the same lens on the same camera may exhibit different focal lengths, and thus need separate camera calibrations. The easiest solution to this problem is to fix the focus of the lens at infinity and use a small enough aperture small to image the closest objects in the scene in focus." So changing the focus can modify the focal length but if I'm not wrong the relation between focal length and radial distortion is not modified by the focus. Maybe it's to stay as close as possible to the theoretical perspective camera model? In Debevec paper: "Real cameras deviate from the pinhole model in several respects. First, in order to collect enough light to expose the film, light is gathered across the entire surface of the lens. The most noticeable effect of this is that only a particular surface in space, called the focal plane, will be in perfect focus." So maybe the reason is just to maximize the sharpness? I asked all those questions because I like the ambition of lensfun and I would like to use it. But I need to be able to calibrate my cameras if they are not already available (no problem to provide the results!). And I really don't want to rely on a manual step to calibrate them by clicking points as I can have lots of data. So we could write some automatic calibration code using checkerboard based on OpenCV if you agree with the idea. 2015-07-14 20:24 GMT+02:00 Torsten Bronger <[email protected]>: > Hallöchen! > > Fabien Castan writes: > > > I don't see anything to calibrate a camera using a checkerboard, > > which allows a fully automatic process. > > The problem is that lens characteristics depend on focus distance. > We need a calibration with focus at infinity, which can be > reasonably achieved with distances larger than 10m. At the same > time, the highest accuracy provide uninterrupted horizontal features > that span the whole image width. This is why I urge people to take > pictures of modern buildings. > > Regards, > Torsten. > > -- > Torsten Bronger Jabber ID: [email protected] > > > > ------------------------------------------------------------------------------ > Don't Limit Your Business. Reach for the Cloud. > GigeNET's Cloud Solutions provide you with the tools and support that > you need to offload your IT needs and focus on growing your business. > Configured For All Businesses. Start Your Cloud Today. > https://www.gigenetcloud.com/ > _______________________________________________ > Lensfun-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/lensfun-users >
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