Taking a screen shoot at a predefined aspect ratio

[Pascal V 1]

I would like my screen shoot area to be constrained by a predefined aspect ratio (ideally it should be possible to change it and chose between various well known ones). If that is not possible at the screenshoot time, then it should be possible to crop it with cropping tool, but again at a fixed aspect ratio. Is there a hidden trick to do so ? Isn't that a interesting feature to suggest ?