Hi laurentvenet, this isn't distortion at your picture, but falling lines, which can be prevented by exact vertical arrangement of the camera (i.e. with a spirit-level). Without the possibility of shifting the lens (i.e. with a large format camera) it can happen then that not everything is on the picture at a huge motive ;-)
15mm is notorious for falling lines, but it can occur also with 50mm at a smaller degree. I don't have a spirit level also, but at landscapes this is better to handle.
It's difficult to control falling lines with the 15mm viewfinder which is heavily distorting. Distortion is a true optical aberration (lens design flaw) and means vertical looking at an rectangle and seeing a barrel (15mm viewfinder!), or a cushion, which can be measured in % of aberration. The 15mm Heliar has less than 1% distortion which is not visible under normal circumstances.
If the lens is used vertical it has no distortion. However, you can use falling lines (as you did) as an effect.
Another thing which can be a bit annoying at 15mm (expecially at landscapes) is light-falloff. This is also not a failure of the lens (or at least to a minor degree). The optics telling the truth that the whole angle of view isn't evenly illuminated, in particular at bright sunshine. The human brain overrules the eye telling sky has to be consistent blue. If you make a picture of the whole scene (11x the format of a standard lens!) pressing it on a 4x5" paper you sometimes get strange results...
Using that 15mm for 4 years I got familiar with this things. 15mm sometimes is a bit exaggerated, even for panoramas.
cheers, Frank
