[CREATE] Lens correction database

[Andrew Zabolotny]

From Thu, 10 Jan 2008 12:04:07 +0100
Pablo dAngelo <Pablo.dAngelo at web.de> wrote:

> > Lens calibration is the main thing still missing in lensfun.
> > Without it there's no way to extend lensfun database, except manual
> > work with MathLab/Octave and alike... and it's a time-consuming
> > thing.
> Hugin is able to provide the radial distortion correction parameters,
> also based with a single photo with straight lines inside.
Indeed, I forgot about Hugin :)

> 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.

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). This approach has also the advantage that the
current database won't have to be modified, just that it'll be a little
inexact.

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 :-)

> 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 ;-)

> 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?

-- 
Andrew
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