Hi Andrew, Andrew Zabolotny wrote:
> > One problem with the "sheet calibration" approach is that it does > > not reflect the typical shooting situation, where the objects are > > (relatively) far away from the camera. The distortion is also focus > > dependent, this effect is stronger if the object is very close to the > > camera (think A4 calibration sheet). A general rule of the thumb is > > to calibrate in a similar distance as the final range. > Indeed, focal length slightly changes with the focus. That should be > the reason why distortion slightly changes too. > > For now lensfun assumes that distortion depends only on the focal > length. I'm not sure how to handle this slight deviation of the focal > length with focus. Either the calibration should be tied to the "true" > focal distance, or calibrate by two factors - focal distance and > distance to subject. Actually, my suggestion would be to just ignore this issue, and calibrate the lens at a distance > 1 m. A few months ago I started evaluate the influence of the object distance on the distortion with several of my lenses, but unfortunately got sidetracked by other stuff. If I remember correctly, the influence was only really visible at very close object distances. > The first approach seems more correct, but it's a question how to > compute the "true" focal distance. Maybe this deviation can be > calibrated too. Then the "true" focal distance (and FoV) could be > computed from the focal distance recorded in EXIF, and the > distance-to-subject, which user will have to enter anyway (for > vignetting correction). Hmm, does the vignetting really change noticeably with varying focus? I haven't made any analysis in that direction yet. > This approach has also the advantage that the > current database won't have to be modified, just that it'll be a little > inexact. My feeling is that introducing such a focal distance vs. focus formula or even calibration is not worth the extra effort. Especially given that the EXIF focal length itself is usually not that accurate either. I guess we should start with what we have now, and get it to a stage where it can be actually used. > If this will be done, it won't depend if you shoot the target at 1m or > at infinity. But this rises the question how to calibrate the focal > distance deviation dependence of the distance-to-subject. > Making a 360 degrees panorama with a 300mm lens is a PITA :-) The Zhang algorithm, for example as implemented in Bouguet's matlab toolbox or the CamChecker program are suitable for this, and will likely provide higher quality estimates as a 360°deg panorama. > > However, it will definately better than no calibration at all. I > > don't believe the vignetting can be recovered faithfully from it > > though. > For vignetting we have PTLens anyway ;-) PTLens is not open source and when I last looked, PTLens only offered an interactive vignetting correction. > > Btw. after the most pressing issues with hugin have been fixed and a > > new stable version is released, I'll look into adding an automatic > > "submit calibration to lens database" feature. > Sounds great. Do you mean to use the PTLens database, or lensfun? The idea is to collect the data, possibly refine it and then include it in lensfun. I have no connections to PTLens, and as you know, PTLens is a closed, commercial product now. Btw. how is progress on the ufraw integration? ciao Pablo _____________________________________________________________________ Der WEB.DE SmartSurfer hilft bis zu 70% Ihrer Onlinekosten zu sparen! http://smartsurfer.web.de/?mc=100071&distributionid=000000000066 _______________________________________________ CREATE mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/create
