Distortion correction [LWN.net]

Distortion correction

Posted Sep 8, 2009 20:21 UTC (Tue) by man_ls (subscriber, #15091)
Parent article: The Grumpy Editor's hugin experience

Distortion correction is a very interesting area, and it is growing more interesting each day. Consider new cameras like the interesting Olympus E-P1: it has pretty sucky optics, but performs distortion correction in software. Other makers are integrating correction of aberrations in the software. This has a few interesting consequences:

Makers can spend less in lenses and do difficult things, like ultra-flat optics.
A firmware update can include better compensations, therefore improving your image quality -- like if you bought a better lens.
OTOH bugs in firmware can mean bad image quality in certain lenses (see the 17mm f2.8 fiasco).
If you export images in RAW format (i.e. without transforming into JPEG), you get the original with all the distortions.
Since only the Olympus software has the parameters to do the corrections, third-party image editors (i.e. those available for Linux) will leave ugly distortions.

This last point is where software like hugin can find another niche: photographers wanting to do their own corrections to pictures. Of course it is much easier to do them in-camera, since you have access to raw pixels, and they are not trivial transformations -- distortions can be compensated with just the kind of thing our editor has tried here, but chromatic aberration correction requires separating components, correcting for different distortions for each color, then mixing it all again. But it is a start.

(Log in to post comments)

Distortion correction

Posted Sep 9, 2009 14:16 UTC (Wed) by kenmoffat (subscriber, #4807) [Link]

Doh! Thanks for that. I use an Olympus E510 - last year I eventually became aware of barrel distortion in some of my photos taken with the 18-42 at wider-than-standard (<25mm). I managed to adequately correct some of them in the gimp (apply negative lens distortion), but my attempts to fix others didn't look better or worse, just different.

Meanwhile, now I was aware of the problem I took some more pictures, using the older, and bigger, 18-45 from my E500. In those I became aware of the distortion in-the-viewfinder on some pics, which is very disconcerting.

After that, I took a series of pics with all my lenses at various lengths and apertures. Used 'display' from ImageMagick to quickly review these, but I couldn't see _any_ significant distortion. By now, my brain was starting to hurt, and I began to doubt my eyesight (and discarded those test pics).

Now I've read your comment, and compared one pic where I remembered mild barrel distortion - sure enough, the raw (.orf) shows it, but olympus's own jpeg is corrected. Thanks again, I'm off to take some more test pics.

ken

Distortion correction

Posted Sep 17, 2009 6:09 UTC (Thu) by eduperez (guest, #11232) [Link]

Only that all those corrections reduce the quality of the image: as you move pixels all around the image, some of them do not fit exactly into their final destination, and must be interpolated; even a simple horizon correction thrashes away some quality.