D'après les trivias d'IMDB :

The noticeably distorted shots throughout the film are the results of Steven Soderbergh placing wider Kowa Anamorphic lenses, which already have considerable barrel distortion on their own, on a large Vista Vision-sized digital sensor that's 1.5x the size of the format they were initially designed for, as he wanted to replicate the imperfect lens characteristics and look of the early anamorphic lenses from the era the film takes place in. In fact, the film has the unusual aspect ratio of 2.16:1 because it was initially planned to be framed in the more traditional 2.40:1 anamorphic ratio, but the barrel distortion on some of those shots ended up being so distracting that he re-framed the entire movie to the taller frame to make it less noticeable.

Merci beaucoup, Olorin !

"less noticeable", je veux bien, mais ça reste quand même très noticeable .