Il me semble que les defaut sur les cotés sont plus lié a des probleme de distortion / aberrations de l'objectif que du temps de pose

voir le chapitre "Lens Aberrations" là

https://improvephotography.com/46266...y-photography/

The Canon 16-35mm F/2.8 has long been Canon’s full-frame, ultra-wide angle workhorse lens. It provides a great focal range for landscape shooting, boasts one of the best sun stars in the business and admirable sharpness, and is a great asset at events such as weddings. For Milky Way photography in particular, the lens is very capable, but does suffer from a bit of coma and sagittal astigmatism in the corners of the frame when shot at its widest apertures. If you have either the version II or version III of this lens, you won’t be limited in capturing a great photo of the Milky Way, just keep in mind that if sagittal astigmatism bothers you, you may have a bit of cleanup to do in post-processing.