Hallöchen! Erik Krause writes:
> Am 03.04.2014 14:33, schrieb Torsten Bronger: > >> AFAIKS, everything is fine. Hugin's calculations are precise. >> The only issue is that Hugin defines a focal length different >> from the one that an optics textbook would define, that's all. > > Optics textbook definitions are simplified, they deal with ideal > lenses. Real lenses suffer from distortions which effectively > result in a different magnification throughout the image (modeled > by the hugin / panotools distortion formula). F.e. higher > magnification in the image center means barrel distortion. Focal > length and magnification are simply related, so there is no > precise single focal length for most real life lenses. Focal length is a matter of definition. And the only definition that is independent of the viewframe is refractive behaviour (magnification) close to the optical axis (paraxial), where a = tan a = sin a = 2 tan a/2 = 2 sin a/2, i.e. the projection formula doesn't matter anymore. Equally, a distortion model like r' = r (ar^3 + br^2 + cr + 1) wouldn't matter either. But as I said, Hugin additionally has this "-a-b-c" term, which changes things. It mathematically transfers part of the paraxial focal length into the distortion parameters, thus resulting in a different focal length. > This is most significant for fisheye lenses. The popular Samyang > 8mm lens f.e. is even marketed as different focal length. You only > can give an approximate value after you estimated the mapping > formula: > http://michel.thoby.free.fr/SAMYANG/Early%20test%20report.html In this link, in the first plot, the focal length is the slope of the lines at the origin. BTW, I own the Samyang 8mm f/2.8, with a projection between equidistant and stereographic. Here's the result of measurement: assumed stereographic a = 0.02036 b = -0.08028 c = 0.01446 FOV = 144.49° Hugin measures a focal length of 8.04mm, but it is 8.4mm actually. Due to distortion, the lens can see more than a perfect stereographic lens should see. I think this is the reason why Samyang markets it with a too short focal length. This may also mean that Hugin's pooly defined focal length (in sense of paraxial optics) blends nicely with manufacturers' habits, so it is probably wise to leave it anyway. But I don't have much data. I owned only one other fisheye, for which a+b+c vanished, so it was not helpful in this regard. ;-) Tschö, Torsten. -- Torsten Bronger Jabber ID: [email protected] or http://bronger-jmp.appspot.com -- A list of frequently asked questions is available at: http://wiki.panotools.org/Hugin_FAQ --- You received this message because you are subscribed to the Google Groups "hugin and other free panoramic software" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/hugin-ptx/87lhvgqqe5.fsf%40physik.rwth-aachen.de. For more options, visit https://groups.google.com/d/optout.
