From what I see, there’s a difference between crop and constrain to image.
If distortion is activated, an optical correction is applied.
Cropping means just cutting out the black parts. The image gets smaller.
Constrain to image means enlarge the image so that the black parts are out of the image. Due to the fact that the heigth and the width aren’t equal the aspect ration is altered a little.
Keep aspect ratio, well just means keep aspect ratio. The size of the image is the original size with the same aspect ratio.
It might help if you watch the move/zoom window. There you can see it happening.
George