Why you should correct for barrel distortion

Why you should correct for barrel distortion
Originally uploaded by Seb Przd.

All zoom lenses are prone to a type of distortion known as barrel distortion, where straight lines appear curved as if the center of the image had been inflated. This is especially the case with the less expensive zoom lenses; usually more expensive lenses, lenses with less zoom range and prime (fixed) lenses are corrected and are closer to a perfcet rectilinear lens. My new lens, the Nikkor 18-200mm, shows a lot of barrel distortion at 18mm.

Fortunately, quite a few tools are available to correct for this predictable type of distortion. I use clens, a command-line utility distributed with panotools, with the lens profiles that come with PTlens, a windows alternative to clens (you can find the database in panotools or here).

The top image above is a straight grid, as shot. You can see the curves that should be straight. The bottom image shows the automatic correction with clens, with the lens profile from PTlens.

clens is run like this:

clens -p profile.txt -s nona -l "Nikkor 18-200mm f/3.5-5.6 G ED-IF AF-S VR DX" image.jpg

If it run like this:

clens -p profile.txt -s nona --Lenses image.jpg

it outputs a list of possible lenses, according to the make of the camera and the focal length.
The correction is not perfect, since the Nikkor 18-200mm shows a more complicated (higher order) type of distortion that clens cannot correct for.

Turkish Delight

Turkish Delight
Originally uploaded by Seb Przd.

Having fun with conformal mappings.

Honey I Escherized the kids!

Honey I Escherized the kids!
Originally uploaded by Seb Przd.

Conformal mappings are great: it's a very powerful branch of mathematics, which basically says you can deform almost any 2D object and transform it into something else while keeping the shape of objects the same (on a small scale). Hence this picture: you see no holes, no obvious distortion, but this image cannot be real. This trick is inspired by a painting by M.C. Escher: some Dutch mathematicians have found what the transformation is and explained in (almost) layman terms.
This image was done in The Gimp and transformed with MathMap.

Shaft the tilt-shift lenses

Why do you still need tilt-shit lenses with digital cameras ?


The first image was shot "as is" of the Saint Ambroise church in Paris with a wide-angle lens. The second one is the same one, but corrected for perspective distortion with hugin.

Equirectangular obsession

Bois de Vincennes (3)
Originally uploaded by Seb Przd.

I've been going through a full immersive panorama phase recently that has me going shooting many pictures from one spot in all directions. I've even bought a new ultra-wide-angle lens (Canon EF-S 10-22mm) so that I have to take fewer pictures. I'm waiting for my spherical tripod head to arrive (Nodal Ninja). You can see more of my pictures here.

The European Google has no .com website

The european answer to Google? it's called Quaero (I search, in Latin). Quaero will be unveiled in January. I have the awful impression that it's full of hot air ("Say goodbye to text-based search, and welcome multimedia search"). Anyway, the funniest part is that they don't even have the domain name http://www.quaero.com ("High Performance Marketing !"), registered to QUAERO CORP., a North-Carolina company... I think Google may not be really scared by the competition. quaero.net is registered to someone not far from my house. quaero.org redirects to the official website of Quaero, the search engine. It is registered to Vecsys-Research, leading edge speech processing technology.

What do you do when your wine gets old?

The answer, if the wine is from one of the best years of the past century, if it is from a reputed château, if it costs between $1500 and $2000 a bottle, and if you have 500 bottles of it, seems to be that you have to change the cork every 40 years or so to prevent the cork from going mouldy. This is what Stanley Ho, a casino magnate in Macau, has just done.
According to this AFP story (in French, sorry), wine experts from Château Palmer travelled to Macau to open, taste and change the corks in 490 bottles of the 1961 Médoc, a grand cru classé. 1961 is one of the best years (as the drought reduced the output of wine to about a third of the normal years). This operation increases the remaining lifespan of the bottles. With a $1,000,000 collection, it is probably worth it...
In the specially weather controlled room of the Lisboa hotel and casino, the French wine experts delicately opened the bottles, fished out with a small net the small cork pieces that tend to fell out as the cork ages, tasted the wine to determine if the wine matured correctly, and recork it with the special machine Château Palmer has used for the past 50 years. The new corks will be labelled "Rebouché en 2005".
It was estimated that as much as 25 bottles would have to be discarded, but after the event only 5 bottles were deemed to have gone bad.